System and method for near-field electromagnetic ranging

ABSTRACT

A system for measuring distance between a first locus and a second locus includes: (a) at least one beacon device; a respective beacon device of the at least one beacon device being situated at the first locus and transmitting a respective electromagnetic signal; and (b) at least one locator device; a respective locator device of the at least one locator device being situated at the second locus and receiving the respective electromagnetic signal. The respective locator device is situated at a distance from the respective beacon device within near-field range of the respective electromagnetic signal. The respective locator device distinguishes at least two characteristics of the respective electromagnetic signal. The respective locator device employs the at least two characteristics to effect the measuring.

[0001] This application claims benefit of prior filed copendingProvisional Patent Application Serial No. 60/404,602, filed Aug. 19,2002, and copending Provisional Patent Application Serial No.60/404,604, filed Aug. 19, 2002, 2002.

BACKGROUND OF THE INVENTION

[0002] The present invention relates generally to measurement ofdistance or ranging by exploitation of near-field electromagneticbehavior and especially to a system and method for evaluating a distancebetween a transmitter or beacon and a receiver or locator. Still morespecifically, the present invention describes a means for determining arange to a transmit-only beacon without requiring synchronization, andwithout relying on variation in signal amplitude. The present inventionmay be advantageously used as part of a more general system fordetermining position (range and bearing) or for tracking (determiningposition in near real time).

[0003] Related Art

[0004] A variety of techniques are known in the art for usingelectromagnetic signals to determine direction and distance. Thesetechniques are sometimes referred to as radio direction finding andradio ranging. A good summary of the state of the art in radio directionfinding is provided by Jenkins. [Small-Aperture Radio Direction-Finding,by Herndon H. Jenkins; Artech House, Boston; 1991; pp. 1-23.]

[0005] Time Difference and Phase Difference Angle of Arrival

[0006] One technique for radio direction finding has been dubbed timedifference of arrival (TDOA). This technique uses a pair of co-polarizedantennas separated by a baseline distance. An incoming signal incidentin a direction perpendicular to the baseline is received by bothantennas at the same time. When the direction of incidence is notperpendicular to the baseline, one antenna will receive the signalbefore the other. The difference in the time of arrival of the signalsat each antenna can be related to the angle of incidence. Equivalently,this difference in time of arrival may be treated in a manner similar tophase difference. Using this technique, the direction of arrival of anincident plane wave may be determined. This TDOA technique may begeneralized to apply to a network of receiving antennas at knownpositions. By comparing the times of arrival of the signal at eachreceiving antenna, the direction of the incident plane wave may bedetermined. In many (but not necessarily all) circumstances, thedirection from which the incoming plane wave arrives is the direction inwhich a target transmitter resides. Early examples of such radiodirection finding systems include the direction finding systemsdisclosed by J. S. Stone (U.S. Pat. No. 716,134; U.S. Pat. No. 716,135;U.S. Pat. No. 899,272; U.S. Pat. No. 961,265) and Roos (U.S. Pat. No.984,108). Phase detection for angle of arrival in the manner nowgenerally understood in the current art was disclosed by Fritz (U.S.Pat. No. 2,160,135) by Runge (U.S. Pat. No. 2,234,654) and by Budenbom(U.S. Pat. No. 2,423,437). 3-D radio direction finding using phasedifference was disclosed by Jansky (U.S. Pat. No. 2,437,695). Lioio etal (U.S. Pat. No. 5,724,047) disclose a phase and time difference radiodirection finding system.

[0007] Antenna Pattern Angle of Arrival

[0008] Another technique for radio direction finding involves using anantenna whose response varies as a function of angle. In oneimplementation, one might use a directive antenna with a relativelynarrow beam-width in a particular boresight or direction of maximumsignal strength. The orientation of the antenna is varied until thereceived signal is maximized so that the boresight of the antenna isaligned with the direction of the incoming signal. In an alternateimplementation, one might use an antenna with a null in a particularnull direction or direction of minimum signal strength.

[0009] In one early invention, Erskine-Murray et al (U.S. Pat. No.1,342,257) disclose the use of a loop antenna rotated about an axislying in the plane of the loop. A similar apparatus that allowed findinga minimum or null while still receiving a signal was disclosed byRobinson (U.S. Pat. No. 1,357,210). Two loop antennas with orthogonalaxes may be electrically combined so as to create a virtual antennaoriented in the direction of a signal maximum (or minimum). A capacitivecombining arrangement or goniometer was disclosed by Bellini (U.S. Pat.No. 1,297,313), and a transformer or inductive coupling goniometer wasdisclosed by Goldschmidt et al (U.S. Pat. No. 1,717,679). Anelectrically small loop and an electrically small whip (or dipole)antenna may be combined to yield a cardiod type pattern with a sharpnull in a particular azimuthal direction. The orientation of the antennamay be varied until the received signal is minimized, then the nulldirection is aligned with the direction of the incoming signal. Examplesof this technique are disclosed by Taylor (U.S. Pat. No. 1,991,473),Bailey (U.S. Pat. No. 1,839,290), and Busignies (U.S. Pat. No.1,741,282). The technique of goniometer combination of signals fromdirective antennas was also disclosed by Fischer (U.S. Pat. No.2,539,413).

[0010] Amplitude Comparison Angle of Arrival

[0011] Still another technique for determining the angle of arrival of aradio wave is amplitude comparison angle of arrival. The signalamplitudes of two or more antennas are compared so as to determine angleof arrival. For instance if a first antenna signal amplitude is verylarge and a second antenna signal amplitude is small, one can infer thatthe radio wave arrived from the direction of the first antenna's patternmaximum and the second antenna's pattern minimum. If the signals are ofcomparable size, then the radio wave may have arrived from a directionin which the two antennas' patterns have comparable directivity. This issimilar to the traditional goniometer angle of arrival technique alreadymentioned. Examples of this technique include disclosures by Earp (U.S.Pat. No. 2,213,273), Wagstaffe (U.S. Pat. No. 2,213,874), Budenbom (U.S.Pat. No. 2,234,587), and Clark (U.S. Pat. No. 2,524,768).

[0012] Doppler Angle of Arrival

[0013] Yet another technique for radio direction finding takes advantageof the Doppler-Fizeau effect. If a receive antenna is rotated at highspeed about an axis perpendicular to the direction of an incomingsignal, then that incoming signal will be shifted up in frequency as thereceive antenna moves toward the direction of the incoming signal anddown in frequency as the receive antenna moves away from the directionof the incoming signal. In practice, it is not feasible to rotate anantenna at a high enough angular velocity for this effect to be readilyobservable. Instead, a number of receive antennas may be placed in acircle and sequentially scanned or sampled at a high rate in order tosimulate rotation. Such systems were disclosed by Earp (U.S. Pat. No.2,651,774) and Steiner (U.S. Pat. No. 3,025,522).

[0014] Hybrid Angle of Arrival

[0015] The prior art techniques discussed hereinabove for making angleof arrival measurements may be advantageously combined. For example,Edwards et al (U.S. Pat. No. 2,419,946) disclose the combination ofamplitude and phase comparison in a radio direction finding system.Murphy et al (U.S. Pat. No. 5,541,608) disclose combining amplitude andphase comparison in a radio direction finding system. Murphy et al donot employ their disclosed architecture to measure range or distance,and they do not employ near-field behavior of electromagnetic signalingas is taught by the present invention.

[0016] Triangulation

[0017] A variety of radio direction finding measurements from a networkof two or more dispersed positions allows the location of a targettransmitter to be determined. One technique by which this can beaccomplished employs triangulation. For example, if the direction to atarget transmitter has been determined from three known positions, thebearings for the three directions may be plotted on a map, and thelocation of the target transmitter is at the intersection of thebearings, or by the triangular region bounded by the intersections ofthe bearings. An example of such a system was disclosed by Maloney et al(U.S. Pat. No. 4,728,959).

[0018] Radio Ranging

[0019] Radio ranging may be accomplished by triangulation from acollection of direction finding measurements. However, a disadvantage tothis prior art ranging technique is that obtaining even a single rangeor distance calculation requires measurements taken from at least twodifferent positions. The positions must be separated by a baseline thatis a significant fraction of the range to be measured in order to obtaina reliable range determination.

[0020] RADAR

[0021] There are a variety of other ways in which range may be measured.One technique is RAdio Detection And Ranging (RADAR) such as isdisclosed by Plaistowe (U.S. Pat. No. 2,207,267). The radar techniquerelies on the scattering of signals from a target. Radar works well inthe detection of aircraft in an open sky or ships on the surface of anocean but radar detection becomes increasingly difficult when the targetbeing tracked is in a cluttered environment populated by scatterers ofequivalent cross-section to the target one desires to track.

[0022] Passive Tag Ranging

[0023] A passive cooperative target, passive transponder, or passive tagyields better performance than is achieved with an uncooperative radartarget. In a passive tag ranging system, a transmitter radiates a signalthat is received by a passive transponder. The passive transponder takesthe received energy and reradiates the signal. The reradiated signal isreceived at the original transmitter and compared to the originaltransmitted signal. This comparison may involve phase, time delay, orother comparison between the transmitted and received signals whichenables a range measurement. An example of such a system is disclosed byLichtenberg et al (U.S. Pat. No. 4,757,315). A disadvantage of passivetag ranging is that the effective range tends to be relatively short dueto the low power picked up by the tag that is available to bereradiated.

[0024] Active Transponder Ranging

[0025] An active cooperative target is generally more effective inranging operations than a passive target. An active transponder listensfor a particular interrogatory signal and responds with a particularreply signal. The frequency of the reply signal is not necessarily thesame as the interrogatory signal, and the strength of the return signalis not dependent on the strength of the interrogatory signal received bythe target. This technique may be referred to as active transponderranging. The time of flight from an interrogating transmitter to thetransponder and back to a receiver may be determined by a phasecomparison of the original transmitted signal with the signal receivedfrom the remote transponder. In some embodiments, the phase comparisonmay be performed on a modulation imposed on an interrogatory signal anda reply signal. Knowing the wave velocity of signals, the time of flightmay be translated into a distance. Examples of transponder type rangingsystems include disclosures by Green (U.S. Pat. No. 1,750,668), Nicolson(U.S. Pat. No. 1,945,952), Gunn (U.S. Pat. No. 2,134,716), Holmes (U.S.Pat. No. 2,198,113), and Strobel (U.S. Pat. No. 2,248,727). Deloraine etal (U.S. Pat. No. 2,408,048) disclose a system for using time modulatedpulses in a transponder ranging system. Nosker (U.S. Pat. No. 2,470,787)discloses a system for 3-D position measurement using transponderranging, and Williams (U.S. Pat. No. 3,243,812) discloses a particularlysimple transponder system involving cycle counting of a phase comparisonbetween a transmitted signal and a received transponded signal. Adisadvantage of transponder ranging is that it requires an active targetto receive a signal, and transmission of a return signal is generallyinfluenced by some property of the received interrogatory signal.

[0026] Transmit-Only Ranging

[0027] A simpler transmit-only ranging scheme uses a transmit-onlytarget. One way to implement a transmit-only ranging system is tomeasure the amplitude of signals received from a transmitter of knowntransmit power. This amplitude ranging method of radio ranging wasdisclosed by de Forest (U.S. Pat. No. 749,436; U.S. Pat. No. 758,517;U.S. Pat. No. 1,183,802). In some cases the amplitude decreases in apredictable fashion with distance from the receiver. For instance infree space, received power varies as the inverse square of the distance.Knowing the transmit power, the receive power and the properties of theantennas, one can infer the range using a known relationship, such asFriis Law.

[0028] The relationship between transmitted power (P_(TX)) and receivedpower (P_(RX)) in a far-field RF link is given by Friis Law:$\begin{matrix}{P_{RX} = {P_{TX}\frac{G_{TX}G_{RX}\lambda^{2}}{4\quad \pi^{2}r^{2}}}} & \lbrack 1\rbrack\end{matrix}$

[0029] where G_(TX) is the transmit antenna gain,

[0030] G_(RX) is the receive antenna gain,

[0031] λ is the RF wavelength, and

[0032] r is the range between the transmitter and receiver.

[0033] Power rolls off (i.e., power decreases as range increases) in thefar-field as the inverse square of the distance$\left( \frac{1}{r^{2}} \right).$

[0034] Near-field links do not obey this relationship. Near-field powerrolls off at powers higher than inverse square, typically inverse fourth$\left( \frac{1}{r^{4}} \right)$

[0035] or higher.

[0036] This near-field behavior has several important consequences.First, the available power in a near-field link tends to be much higherthan would be predicted from the usual far-field, Friis Lawrelationship. This results in a higher signal-to-noise ratio (SNR) and abetter performing link. Second, because the near-fields have such arelatively rapid power roll-off, range tends to be relatively finite andlimited. Thus, a near-field system is less likely to interfere withanother RF system operating outside the operational range of thenear-field system.

[0037] Inferring range from received signal power or amplitude isproblematic at best. Despite the difficulties, amplitude ranging systemsare still used. For instance Moulin (U.S. Pat. No. 5,955,982) discloseda method and device for detecting and locating people buried under anavalanche in which signal amplitude is used to localize an avalanchevictim.

[0038] There are a variety of other ways by which a receiver can obtainrange information from a transmit-only target. Ranger (U.S. Pat. No.1,639,667) disclosed the idea of synchronized oscillators at atransmitter and at a remote receiver. A receiver can compare the numberof 360° phase shifts or the number of beats per time to infer a changein distance. In a series of inventions, Gage (U.S. Pat. No. 1,828,531;U.S. Pat. No. 1,939,685; U.S. Pat. No. 1,939,686; U.S. Pat. No.1,961,757) disclosed transmitting a pair of signals at differentfrequencies with different propagation characteristics and differentattenuation constants. By comparing the amplitude ratio of the receivedsignals, range may be inferred. Runge (U.S. Pat. No. 2,134,535)disclosed looking at the superposition of direct and reflected rays in areceived signal to infer range from a transmitter. Herson (U.S. Pat. No.2,314,883) disclosed evaluating the rate of change of the amplitude of areceived signal in order to infer range. Hammerquist (U.S. Pat. No.4,788,548) disclosed a multi-channel receiver for making phasemeasurements that allows a range measurement to be made. More recently,Sullivan (U.S. Pat. No. 5,999,131) disclosed a network of receiversisolating the direct path signal from a transmitter. Relative phasedifference measurements between receivers in the network are convertedinto differential range estimates for locating the transmitter.Sullivan's system has the disadvantage of requiring a common time baseor synchronization among all receivers in the network.

[0039] If a transmitter and a receiver are synchronized, then a precisephase measurement at a receiver can yield range information, up to a360° phase uncertainty. In other words, a synchronized receiver candetermine the location of a transmitter relative to the start and end ofa wavelength, yet not be able to determine whether the transmitter'sposition lies within (for example) the seventh, or eighth or some otherwavelength away. If the transmitter's absolute (or reference) positionis determined initially by some other means, then the receiver can trackthe change in position of the transmitter relative to the establishedreference. Precise synchronization is essential to achieving meaningfulrange information in such a system. Any clock drift between thetransmit-only target and the receiver results in a range error. As apractical matter, however, precise synchronization is exceedinglydifficult and often expensive to achieve.

[0040] Transmit-only ranging may also be accomplished with anunsynchronized transmit-only target, using a network of synchronizedreceivers. The relative difference in received phase can be translatedinto a relative difference in position, subject to a 360° phaseambiguity.

[0041] All of these transmit-only ranging schemes rely on the“far-field” assumption: one must assume that a transmit-only target anda receiver are located at least a half wavelength apart. If atransmit-only target and a receiver are located within a half wavelengthor less of each other, then near-field ambiguities make it difficult todetermine an accurate range.

[0042] The simplicity of a transmit-only ranging system is attractive.However existing transmit-only ranging systems suffer from significantdisadvantages. Some transmit-only ranging systems are dependent onprecise synchronization of a network of receivers that tend to becomplex, difficult, and expensive to implement. Some transmit-onlyranging systems are dependent on measurement of a precise time betweentransmission and reception in order to calculate distance by multiplyingtime and signal velocity. Some transmit-only ranging systems aredependent on a similarly difficult synchronization of transmitter andreceiver. Some transmit-only ranging systems are dependent oncalibrating a transmitter to a known position before an absolute rangecan be determined. Some transmit-only ranging systems are dependent on apredictable variation between range and amplitude seldom found in thereal world.

[0043] As far as the inventors are aware, prior art electromagnetictracking and ranging systems are dependent on far-fields: radiatedelectromagnetic fields received at distances on the order of awavelength or (usually) much further. Even inventors such as Ranger(U.S. Pat. No. 1,639,667) who disclose operation at ranges on the orderof a wavelength or less implicitly assume far-field signal behavior. Noprior art ranging system known to the inventors exploits near-fieldsignal phenomena in performing ranging or distance measurement. Thepresent invention employs near-field signal phenomena to advantage andhas none of the dependencies and shortcomings noted in prior art rangingsystems.

[0044] Historical Context

[0045] Some of the earliest wireless communication systems involvednear-field or inductive coupling. One example involved couplingtelegraph signals between a moving train and an adjacent telegraph line.With the discoveries of Hertz (as put into practice by such innovatorsas Marconi, Lodge, and Tesla), the overwhelming emphasis of RFdevelopment focused on long range, far-field systems. Frequencies wererelatively low by contemporary standards. The earliest development wasin the low frequency (LF) band (30 kHz-300 kHz), and soon progressed tothe medium frequency (MF) band (300 kHz-3 MHz), with some pioneeringwork extending into the high frequency (HF) band (3-30 MHz). This workwas oriented toward the empirical. Engineers focused on practicaltechniques for radiating and receiving signals. Little work was done todefine or understand the fundamental physics that enables the radiofrequency (RF) arts. For example, in 1932 the eminent RF expert,Frederick Terman, could say, “An understanding of the mechanism by whichenergy is radiated from a circuit and the derivation of equations forexpressing this radiation quantitatively involve conceptions which areunfamiliar to the ordinary engineer.” [Radio Engineering, First Edition,by Frederick Emmons Terman; McGraw Hill, Book Co., Inc., New York; 1932;p. 494.] At that time the frontier of the RF arts had just begun toprobe the lower end of the very high frequency (VHF) band (30 MHz-300MHz). One textbook from the period provides a spectrum chart that endswith “30,000 kHz-60,000 kHz: Experimental and Amateur; >60,000 kHz: NotNow Useful”. [Radio Physics Course, Second Edition, by Alfred A.Ghirardi; Farrar & Rinehart, Inc., New York; 1942; p. 330.] Radiodirection finding and ranging remained focused on long range, far-fieldapplications such as radio navigation for airplanes and radio guidancesystems. The Japanese homed in on a Honolulu radio station in theirattack on Pearl Harbor [Joe Carr's Loop Antenna Handbook, First Edition,by Joseph J. Carr; Universal Radio Research, Reynoldsburg, Ohio; 1999;p. 85.] Only in the 1940's, with the development of RADAR, did atheoretical emphasis in the RF arts catch up with the longstandingempirical emphasis. By then however, the RF frontier had rapidly passedthrough VHF and UHF and moved on to microwaves. The LF, MF, and even theHF bands were increasingly a backwater far removed from the activeattention of most RF engineers.

[0046] In short, by the time fundamental electromagnetic theory began tobe actively applied by RF engineers, RF engineers were not activelyfocused on applying this theory to the problem of radio ranging at lowfrequencies such as those in the LF, MF, and HF band. By and large, theoverwhelming emphasis in the RF arts has been toward far-field systems,ones that operate at ranges beyond a wavelength, rather than near-fieldsystems that operate at ranges within a wavelength or so.

[0047] Lower frequencies have certain advantages over higherfrequencies. Lower frequencies tend to diffract better aroundobstructions and thus can be used in non-line-of-sight applications suchas over a hill or around a building. Because of the longer wavelengthsassociated with lower frequencies, multipath interference is far less ofa problem than at higher frequencies. Further, lower frequencies tend tobe more penetrating of foliage and typical building materials, such aswood, brick, or concrete. Lower frequency RF circuits tend to be easierto build, and more robust. Components for use at lower RF frequenciestend to be less expensive and more readily available than those for useat higher frequencies.

[0048] Operation in the near-field, at ranges within a wavelength or so,yields certain advantages as well. Near-field signal levels tend to befar higher than would be predicted from the usual inverse range square$\left( \frac{1}{r^{2}} \right)$

[0049] far-field radiation relationships. In contrast, signal levels inthe near-field decrease more rapidly than in the far-field, decreasingin intensity as a function of $\frac{1}{r^{4}}.$

[0050] As a result, there is a lesser problem with electromagneticinterference among adjacent near-field systems so that it is easier tore-use the same frequency in a smaller cell size than would be expectedfrom the usual far-field predictions. In short, electromagnetic wavesbehave differently in the near-field than in the far-field, and theinventors have discovered that the continuous and predictable variationof certain electromagnetic parameters may be used as signals traversethe near-field en route to the far-field to ascertain range or distanceinformation.

[0051] Despite these near-field advantages, to the best of the knowledgeof the inventors, no prior art describes a system in which near-fieldsignal phenomena and the predictable behavior of those phenomena as theytransition from near-field to far-field behavior are exploited in orderto obtain range or distance information.

[0052] There is a need for an electromagnetic ranging apparatus andmethod that can be operated asynchronously without requiringsynchronization of transmitter to receiver or synchronization among anetwork of receivers.

[0053] There is a further need for an electromagnetic ranging apparatusand method that can be operated without an awkward and lengthycalibration process and that can be useful in a wide variety ofpropagation environments.

[0054] There is another need for an electromagnetic ranging apparatusand method that can be used as part of a location or position trackingsystem.

[0055] There is yet a further need for a system and method for findingthe range to or position of a source of electromagnetic signals whoselocation is unknown.

[0056] There is still a further need for a system and method ofelectromagnetic ranging that operates using relatively low frequenciesand takes advantage of the characteristics of near-fields.

SUMMARY OF THE INVENTION

[0057] A system for measuring distance between a first locus and asecond locus includes: (a) at least one beacon device; a respectivebeacon device of the at least one beacon device being situated at thefirst locus and transmitting a respective electromagnetic signal; and(b) at least one locator device; a respective locator device of the atleast one locator device being situated at the second locus andreceiving the respective electromagnetic signal. The respective locatordevice is situated at a distance from the respective beacon devicewithin near-field range of the respective electromagnetic signal. Therespective locator device distinguishes at least two characteristics ofthe respective electromagnetic signal. The respective locator deviceemploys the at least two characteristics to effect the measuring.

[0058] A method for measuring distance between a first locus and asecond locus includes the steps of: (a) transmitting an electromagneticsignal from the first locus; (b) receiving the electromagnetic wave atthe second locus; the second locus being within near-field range of theelectromagnetic signal; (c) in no particular order: (1) detecting afirst characteristic of the electromagnetic signal; and (2) detecting asecond characteristic of the electromagnetic signal; (d) measuring adifference between the first characteristic and the secondcharacteristic; and (e) employing the difference to calculate thedistance.

[0059] The electromagnetic ranging apparatus of the present inventionemploys near-field electromagnetic behavior to measure a distancebetween a transmit beacon and a receive locator. A locator includes: (a)a first receiving antenna sensitive to electric (E) fields, (b) a secondreceiving antenna sensitive to magnetic (H) fields, (c) a means forreceiving a first signal from a first receiving antenna, (d) a means forreceiving a second signal from a second receiving antenna, (e) a meansfor determining a difference between a first signal and a second signal,and (f) a means for determining a distance of a beacon from a locatorusing a difference. The present invention demonstrates that a phasedifference between electric and magnetic fields may be exploited todetermine a range to a beacon, such as a transmitter or other source ofelectromagnetic waves or signals. Typical implementations can determinea range to a beacon between about 0.05 λ and 0.50 λ away, where λ is thewavelength of the electromagnetic signal transmitted by the beacon.Higher performance implementations of the present invention may operateat ranges less than 0.05 λ and greater than 0.50λ.

[0060] It is an object of the present invention to provide anelectromagnetic ranging apparatus and method that can be operatedasynchronously without requiring synchronization of transmitter toreceiver or synchronization among a network of receivers.

[0061] It is a further object of the present invention to provide anelectromagnetic ranging apparatus and method that can be operatedwithout an awkward and lengthy calibration process and that can beuseful in a wide variety of propagation environments.

[0062] Yet another object of the present invention is to provide anelectromagnetic ranging apparatus and method that can be used as part ofa location or position tracking system.

[0063] An additional object of the present invention is to provide asystem and method for finding the range to or position of a source ofelectromagnetic waves whose location is unknown.

[0064] Still another object of the present invention is to provide asystem and method of electromagnetic ranging that operates usingrelatively low frequencies and takes advantage of the characteristics ofnear-fields.

[0065] Further objects and features of the present invention will beapparent from the following specifications and claims when considered inconnection with the accompanying drawings, in which like elements arelabeled using like reference numerals in the various figures,illustrating the preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0066]FIG. 1 is a graphic representation of electric and magnetic fieldphase relationships as a function of range for an ideal electricallysmall loop in free space.

[0067]FIG. 2 is a table relating range of operation and frequency for anear-field ranging system.

[0068]FIG. 3 is a schematic illustration of a system for near-fieldranging by comparison of electric and magnetic field phase inquadrature.

[0069]FIG. 4 is a schematic illustration of a system for near-fieldranging by comparison of electric and magnetic field phase in phasesynchrony.

[0070]FIG. 5 is a schematic illustration of a system for near-fieldranging by comparison of electric and magnetic field phase.

[0071]FIG. 6 is a schematic diagram of details of a preferred embodimentof a system for near-field ranging by comparison of electric andmagnetic field phase.

[0072]FIG. 7 is a schematic diagram of a system for near-field rangingby comparison of electric and magnetic field phase with beacon andlocator function combined in a single unitary device.

[0073]FIG. 8 is a schematic illustration of a representative antennaconfiguration for a near-field ranging system having a verticalpolarization beacon and a vertical polarization omni-directionallocator.

[0074]FIG. 9 is a schematic illustration of a representative antennaconfiguration for a near-field ranging system having a horizontalpolarization beacon and a horizontal polarization omni-directionallocator.

[0075]FIG. 10 is a schematic illustration of a representative antennaconfiguration for a near-field ranging system having a verticalpolarization beacon and a vertical polarization directional locator.

[0076]FIG. 11 is a schematic illustration of a representative antennaconfiguration for a near-field ranging system having a horizontalpolarization beacon and a horizontal polarization directional locator.

[0077]FIG. 12 is a schematic diagram illustrating details of anexemplary receiver in a system for electromagnetic ranging.

[0078]FIG. 13 is a schematic diagram illustrating a near-field rangingsystem configured according to a fixed beacon-mobile locatorarchitecture.

[0079]FIG. 14 is a schematic diagram illustrating a near-field rangingsystem configured according to a fixed/mobile locator-mobile beaconarchitecture.

[0080]FIG. 15 is a schematic diagram illustrating a near-field rangingsystem configured according to a reciprocal beacon-locator architecture.

[0081]FIG. 16 is a schematic diagram illustrating a near-field rangingsystem configured employing a passive tag architecture.

[0082]FIG. 17 is a schematic diagram illustrating a near-field rangingsystem configured employing a near-field remote sensing architecture.

[0083]FIG. 18 is a flow diagram illustrating the method of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0084] Overview of the Invention The present invention will now bedescribed more fully in detail with reference to the accompanyingdrawings, in which the preferred embodiments of the invention are shown.This invention should not, however, be construed as limited to theembodiments set forth herein; rather, they are provided so that thisdisclosure will be thorough and complete and will fully convey the scopeof the invention to those skilled in art. Like numbers refer to likeelements throughout.

[0085] An Analytic Model

[0086] Suppose a transmit-only target uses a small loop antenna thatbehaves like a time domain magnetic dipole. A magnetic dipole may bethought of as a small current loop of area A, and a time dependentcurrent I=I₀ T(t) where I₀ is an initial or characteristic current andT(t) is the time dependence. Assume the dipole lies in the x-y planecentered at the origin with its axis in the z direction. The dipole'smagnetic moment m is: m=AI₀ T(t), or m=m₀ T(t). The magnetic field or“H-field” of this small loop is: $\begin{matrix}{{{H(t)} = {{\frac{m_{0}}{4\quad \pi \quad r^{2}}\left( {\frac{T}{r} + \frac{\overset{.}{T}}{c}} \right)\left( {{2\quad \cos \quad \theta \quad \hat{r}} + {\sin \quad \theta \quad \hat{\theta}}} \right)} + {\frac{m_{0}\overset{¨}{T}\quad \sin \quad \theta}{4\quad \pi \quad c^{2}r}\hat{\theta}}}},} & \lbrack 2\rbrack \\{{{and}\quad {the}\quad {electric}\quad {field}\quad {{or}\quad {''}}E} - {{{field}{''}}\quad {{is}:}}} & \quad \\{{{E(t)} = {{- \frac{1}{4\quad \pi \quad ɛ_{0}}}\frac{m}{c^{2}r}\left( {\frac{\overset{.}{T}}{r} + \frac{\overset{¨}{T}}{c}} \right)\sin \quad \theta \quad \hat{\phi}}},} & \lbrack 3\rbrack\end{matrix}$

[0087] where r is the range from the origin, c is the speed of light, ε₀is the permeability of free space, and derivatives with respect to timeare denoted by dots. Assume a sinusoidal excitation T(t)=sinωt where ωis the angular frequency: ω=2πf. Then, {dot over (T)}(t)=ωcosωt, {umlautover (T)}(t)=−ω²sinωt, $\begin{matrix}{{{H(t)} = {{\frac{m_{0}}{4\quad \pi \quad r^{2}}\left( {\frac{\sin \quad \omega \quad t}{r} + \frac{\omega \quad \cos \quad \omega \quad t}{c}} \right)\left( {{2\quad \cos \quad \theta \quad \hat{r}} + {\sin \quad \theta \quad \hat{\theta}}} \right)} - {\frac{m_{0}\omega^{2}\quad \sin \quad \omega \quad t\quad \sin \quad \theta}{4\quad \pi \quad c^{2}r}\hat{\theta}}}},} & \lbrack 4\rbrack \\{{and}:} & \quad \\{{E(t)} = {{- \frac{1}{4\quad \pi \quad ɛ_{0}}}\frac{m_{0}}{c^{2}r}\left( {{\frac{\omega}{r}\cos \quad \omega \quad t} - {\frac{\omega^{2}}{c}\sin \quad \omega \quad t}} \right)\sin \quad \theta \quad {\hat{\phi}.}}} & \lbrack 5\rbrack\end{matrix}$

[0088] There are a variety of ways in which range information may beobtained from near-fields. For instance, one could compare alongitudinal or radial ({circumflex over (r)}) component of a firstfield to a transverse component ({circumflex over (θ)} or {circumflexover (φ)}) of a first field. One could compare a longitudinal or radial({circumflex over (r)}) component of a first field to a transversecomponent ({circumflex over (θ)} or {circumflex over (φ)}) of a secondfield. One could compare a longitudinal or radial ({circumflex over(r)}) component of a first field to a longitudinal or radial({circumflex over (r)}) component of a first field. One could compare alongitudinal or radial ({circumflex over (r)}) component of a firstfield to a longitudinal or radial ({circumflex over (r)}) component of asecond field. One could compare a transverse component ({circumflex over(θ)} or {circumflex over (φ)}) of a first field to a transversecomponent ({circumflex over (θ)} or {circumflex over (φ)}) of a firstfield. One could compare a transverse component ({circumflex over (θ)}or {circumflex over (φ)}) of a first field to a transverse component({circumflex over (θ)} or {circumflex over (φ)}) of a second field.These comparisons may include comparisons of phase, comparisons ofamplitude, or comparisons of other signal properties.

[0089] The inventors have discovered that one particularly advantageousand useful comparison is a comparison of phase of an electric componentof an electromagnetic wave to phase of a magnetic component of anelectromagnetic wave.

[0090] For this ideal small loop in free space, E-field phase in degreesas a function of range is: $\begin{matrix}{\varphi_{E} = {\frac{180}{\pi}{\left( {\frac{\omega \quad r}{c} + {\cot^{- 1}\frac{\omega \quad r}{c}}} \right).}}} & \lbrack 6\rbrack\end{matrix}$

[0091] Transverse H-field phase in degrees as a function of range is:$\begin{matrix}{\varphi_{H} = {\frac{180}{\pi}{\left( {\frac{\omega \quad r}{c} + {\cot^{- 1}\left( {\frac{\omega \quad r}{c} - \frac{c}{\omega \quad r}} \right)}} \right).}}} & \lbrack 7\rbrack\end{matrix}$

[0092] Note that Equation (6) has a branch cut at a range$r = {\frac{1}{2\quad \pi}{\lambda.}}$

[0093] The phase delta is given by: $\begin{matrix}{\Delta_{\varphi} = {{\varphi_{H} - \varphi_{E}} = {\frac{180}{\pi}{\left( {{\cot^{- 1}\left( {\frac{\omega \quad r}{c} - \frac{c}{\omega \quad r}} \right)} - {\cot^{- 1}\frac{\omega \quad r}{c}}} \right).}}}} & \lbrack 8\rbrack\end{matrix}$

[0094] These relations assume a measurement in the plane of the loop(θ=90°). Similar relations may be derived for other orientations.

[0095]FIG. 1 is a graphic representation of electric and magnetic fieldphase relationships as a function of range for an ideal electricallysmall loop in free space. In FIG. 1, a graphic plot 100 includes amagnetic or H-Field phase curve 102, an electric or E-Field phase curve104 and a phase difference or Δφ curve 106 representing the differencebetween curves 102, 104. Curves 102, 104, 106 are plotted against afirst axis 108 representing phase (preferably in degrees) as a functionof range represented on a second axis 110 in wavelength (preferably in akilogram-meter-second unit, such as meters) of an electromagnetic signalunder consideration. Thus, the relations of Equations [6]-[8] areillustrated in graphical representation 100. H-field phase curve 102,described by Equation [7], begins 90° out of phase with respect toE-field phase 104, described by Equation [6]. As range is increased fromabout 0.05 λ to about 0.50×, H-field phase curve 102 initiallydecreases, then increases. Similarly, as range is increased from about0.05 λ to about 0.50 λ, E-field phase curve 104 increases, gradually atfirst, and at an increasing rate as range increases. The differencebetween E-field phase curve 104 and H-field phase curve 102 isrepresented by Δφ curve 106. Δφ curve 106 begins at approximately 90°(i.e., at phase quadrature) in the near-field within a range of about0.05 λ and goes to 0° (i.e., phase synchronicity) as the far-field isapproached, past a range of about 0.50 λ. Δφ curve 106 is describedmathematically in Equation [8]. Transition of Δφ curve 106 from phasequadrature to phase synchronicity between about 0.05 λ to about 0.50 λis substantially continuous and predictable and is used to advantage bythe present invention. With more precise measurement, this phasetransition can be beneficially used at ranges inside 0.05 λ and outside0.50 λ.

[0096] Equation [8] expresses phase difference Δφ as a function of range(r). Equation [8] is a transcendental relation that may not be invertedto yield an expression for range as a function of phase difference.Nevertheless, a variety of mathematical methods may be used to determinea range given a phase difference. Equation [8] may be advantageouslyemployed by other mathematical techniques such as, by way of example andnot by way of limitation, solving numerically, generating a look-uptable, and solving graphically.

[0097] In the far-field, at distances greater than one wavelength, boththe electric and magnetic fields are phase synchronous. The phase ofeach field varies in lock step with the other field at a fixed rate of360° per wavelength in the far-field limit. This is the usualrelationship expected by those skilled in the RF arts. As a rule, thenear-field phase anomalies exploited by the preferred embodiment ofpresent invention are rarely discussed, if at all, in the prior art. Oneexception to this rule is the work of one of the inventors.[Electromagnetic Energy Around Hertzian Dipoles, by H. Schantz; IEEEAntennas and Propagation Magazine, April 2001; pp. 50-62.]

[0098]FIG. 2 is a table relating range of operation and frequency for anear-field ranging system. In FIG. 2, a table 200 relates frequency withselected ranges expressed in terms of wavelength of a signal underconsideration. An important feature of the present invention is that aphase difference Δφ between electric and magnetic fields may beexploited to determine a range from a locator receiver to a beacontransmitter, or other source of electromagnetic waves. This near-fieldranging method allows a distance to a beacon to be accurately determinedbetween about 0.05 λ and 0.50 λ from the beacon, where λ is thewavelength of electromagnetic signal transmitted by a beacon. Optimumperformance is obtained from a range of about 0.08 λ to a range of about0.30 λ from the beacon. With more precise measurement, this phasetransition can be used for ranges inside 0.05 λ and outside 0.50 λ. Acorresponding characteristic range of operation as a function offrequency is presented in table 200; FIG. 2. Lower frequencies permitoperation at longer ranges; higher frequencies are preferred for shorterranges. The particular frequencies listed in table 200 (FIG. 2) arepresented for purposes of illustration and not for purposes oflimitation.

[0099] Determination of a range from a phase difference αφ between anelectric and a magnetic field may be more complicated than the freespace result of Equation [8] indicates. In practice, one may wish tocalibrate a ranging system using a more complicated analytical orcomputational model (for example, a model including the effect ofpropagation over a real ground instead of free space), or usingexperimental data from an environment within which one wishes to carryout ranging operations.

[0100] The present invention allows ranging to at least 3000 feet in the160-190 kHz band, to at least 900 feet in the AM radio band, and toshorter ranges at higher frequencies. A wide variety of otheroperational ranges are available by using other frequencies. Greaterrange can be achieved with lower frequency. Accuracy within inches isachievable even at the longest ranges.

[0101] In the interest of presenting a simple illustrative example ofthe present invention, that is by way of illustration and not by way oflimitation, this description addresses a mobile beacon and a stationarylocator, but one skilled in the art may easily recognize that a beaconmay be fixed and the locator mobile, or both beacon and locator may bemobile. To avoid unnecessary prolixity in the discussion that follows,sometimes only a single locator and a single beacon are discussed. Thisshould not be interpreted so as to preclude a plurality of beacons andlocators used as part of a more complicated positioning, locating, ortracking system.

[0102] A System For Near-Field Ranging

[0103]FIG. 3 is a schematic illustration of a system for near-fieldranging by comparison of electric and magnetic field phase inquadrature. In FIG. 3, a ranging system 300 is illustrated fornear-field ranging by comparison of electric and magnetic field phasewith the electric and magnetic field signals in quadrature (90° out ofphase) at close range. A beacon 310 includes a transmitter 312 and atransmit antenna 337. Beacon 310 transmits an electromagnetic wave orsignal 315 having a wavelength λ.

[0104] A locator 320 receives electromagnetic signal 315. Locator 320includes a first electric field antenna 332 for receiving an E-fieldsignal 301 and a second magnetic field antenna 331 which receives anH-field signal 302. If a distance 304 between beacon 310 and locator 320is, for example, 0.05 λ, then E-field signal 301 and H-field signal 302are approximately 90° out of phase at antennas 331, 322. Locator 320measures this phase difference Δφ and indicates that distance equals0.05 λ in a distance indicator 306.

[0105]FIG. 4 is a schematic illustration of a system for near-fieldranging by comparison of electric and magnetic field phase in phasesynchrony. In FIG. 4, a ranging system 400 is illustrated for near-fieldranging by comparison of electric and magnetic field phase with theelectric and magnetic field signals in phase synchronicity (0° phasedifference) at far range. A beacon 410 includes a transmitter 412 and atransmit antenna 437. Beacon 410 transmits an electromagnetic signal 415having a wavelength λ.

[0106] A locator 420 receives electromagnetic signal 415. Locator 420has a first electric field antenna 432 which receives an E-field signal401, and a second magnetic field antenna 431 which receives an H-fieldsignal 402. If distance 404 between beacon 410 and locator 420 is 0.50λ, then E-field signal 401 and H-field signal 402 are approximately 0°out of phase (in phase synchronicity). Locator 420 measures this phasedifference αφ and indicates that distance equals 0.05 λin a distanceindicator 406.

[0107] Either locator 320, 420 may use the free space relationshipbetween phase difference Δφ and range r described mathematically inEquation [8], may use a more exact analytic expression taking intoaccount the effects of soil and ground propagation, may use atheoretical simulation of the propagation environment, or may useempirical data regarding phase difference and range in a particularpropagation environment or another basis for determining therelationship between phase difference Δφ and range r.

[0108] Basic Architecture of a System for Near-Field Ranging

[0109]FIG. 5 is a schematic illustration of a system for near-fieldranging by comparison of electric and magnetic field phase. In FIG. 5, aranging system 500 is illustrated for near-field ranging by comparisonof electric and magnetic field phase with the electric and magneticfield signals. A beacon 510 includes a transmitter 512 and a transmitantenna 536. Beacon 510 may be mobile, or fixed, or even an unknown oruncooperative source of electromagnetic radiation in the form of anelectromagnetic signal 515. Transmit antenna 536 can be a loopstickantenna or another type antenna that is substantially unaffected bychanges in an adjacent propagation environment. Transmit antenna 536could also be a whip antenna that is as large as is allowed by eitherpertinent regulations or the constraints imposed by a particularapplication. Beacon 510 transmits electromagnetic signal 515.

[0110] A locator 520 is situated a distance r from beacon 510 andreceives electromagnetic signal 515. Locator 520 includes a firstantenna 531, a first receiver 525, a second antenna 532, a secondreceiver 527, a signal comparator 580, and a range detector 590. Signalcomparator 580 receives a first representative signal from firstreceiver 525 and a second representative signal from second receiver527. Signal comparator 580 receives the first and second representativesignals and identifies a difference between the first and secondrepresentative signals. The identified difference may be a difference inphase, a difference in amplitude, or another difference between thefirst and second representative signals. Signal comparator 580 generatesa third signal proportional to or otherwise related to the differenceidentified by signal comparator 580. Range detector 590 receives thethird signal from signal comparator 580 and employs the received thirdsignal to determine range r between beacon 510 and locator 520.

[0111] In the preferred embodiment of the present invention, firstantenna 531 is configured to permit first receiver 525 to generate thefirst representative signal provided to signal comparator 580 as asignal proportional to or otherwise representative of a first componentof electromagnetic signal 515. Further in the preferred embodiment ofthe present invention, second antenna 532 is configured to permit secondreceiver 527 to generate the second representative signal provided tosignal comparator 580 as a signal proportional to or otherwiserepresentative of a second component of electromagnetic signal 515. Thefirst component and second component of electromagnetic signal 515 maydiffer in polarization or some other detectable property. One differenceadvantageous in a near-field ranging system is a difference between alongitudinal or radial ({circumflex over (r)}) component and atransverse component ({circumflex over (θ)} or {circumflex over (φ)}) ofelectromagnetic signals.

[0112] In another preferred embodiment of the present invention, firstantenna 531 is an electric or E-field antenna that permits firstreceiver 525 to generate the first representative signal provided tosignal comparator 580 as a signal proportional to or otherwiserepresentative of a first component of electromagnetic signal 515, andsecond antenna 532 is a magnetic or H-field antenna that permits secondreceiver 527 to generate the second representative signal provided tosignal comparator 580 as a signal proportional to or otherwiserepresentative of a second component of electromagnetic signal 515.

[0113] In the most preferred embodiment of the present invention, firstantenna 531 is an H-field antenna, first receiver 525 is an H-fieldreceiver, second antenna 532 is an E-field antenna, second receiver 527is an E-field receiver, signal comparator 580 is a phase detector andrange detector 590 employs phase information received from signalcomparator-phase detector 580 to determine range r between beacon 510and locator 520.

[0114] Thus, in the most preferred embodiment of the present inventionfirst (H-field) antenna 531 responsive to a magnetic or H-fieldcomponent of electromagnetic signal 515 and permits first (H-field)receiver 525 to detect a first signal proportional to the magnetic orH-field component of electromagnetic signal 515. Antennas responsive toa magnetic or H-field component of an electromagnetic signal include, byway of example and not by way of limitation, loop and loopstickantennas. First (H-field) receiver 525 receives a signal from first(H-field) antenna 531 and generates a first representative signalproportional to the magnetic or H-field component of electromagneticsignal 515. The representative signal may, for example, be an analogsignal having a voltage that is directly proportional to amplitude ofthe magnetic or H-field component of electromagnetic signal 515.Alternatively, the representative signal may be, for example, a digitalsignal conveying data pertaining to the magnetic or H-field component ofelectromagnetic signal 515. First (H-field) receiver 525 may includefiltering, amplification, analog to digital conversion, and tuning meansof the kind that are generally understood by practitioners of the RFarts.

[0115] Second (E-field) antenna 532 responsive to an electric or E-fieldcomponent of electromagnetic signal 515 allows second (E-field) receiver527 to detect a second signal proportional to an electric or E-fieldcomponent of electromagnetic signal 515. Antennas responsive to anelectric or E-field component of an electromagnetic wave include, by wayof example and not by way of limitation, whip, dipole, or monopoleantennas. Second (E-field) receiver 527 detects an input signal fromsecond (E-field) antenna 532 and yields a second signal proportional tothe electric or E-field component of electromagnetic signal 515. Therepresentative signal may, for example, be an analog signal whosevoltage is directly proportional to amplitude of the electric or E-fieldcomponent of electromagnetic signal 515. Alternatively, therepresentative signal may be, for example, a digital signal conveyingdata pertaining to the electric or E-field component of electromagneticsignal 515. Second (E-field) receiver 527 may include filtering,amplification, analog to digital conversion, and tuning means of thekind that are generally understood by practitioners of the RF arts.

[0116] If electromagnetic signal 515 is a single frequency sine wave, itis desirable for a first (H-field) receiver 525 and a second (E-field)receiver 527 to employ a very narrow bandwidth filter so as to minimizethe noise and maximize the signal to noise ratio. However, it is alsoimportant for filters used in a first (H-field) receiver 525 and asecond (E-field) receiver 527 to have a constant passband group delay sothat relative phase characteristics of a first representative signal anda second representative signal are stable and predictable. The inventorshave advantageously employed Bessel filters as a starting point foroptimization.

[0117] First (H-field) antenna 531 and second (E-field) antenna 532 arepreferably oriented to be maximally responsive to polarization ofelectromagnetic signal 515. In alternate embodiments, locator 520 mayemploy additional (H-field) antennas, additional (E-field) antennas,additional H-field receivers, and additional E-field receivers in orderto detect multiple polarizations or so as to detect electromagneticsignals from additional incident directions. Because electromagneticsignal 515 has near-field characteristics, polarizations mayadvantageously include a longitudinal polarization with a componentparallel to a direction of travel of an incident electromagnetic signal.

[0118] Signal comparator 580 (preferably embodied in a phase detector)takes the first representative signal proportional to the magnetic orH-field component of electromagnetic signal 515 and the secondrepresentative signal proportional to the electric or E-field componentof electromagnetic signal 515 and determines a phase difference betweenthe first and second representative signals. Phase detector 580 may bethought of (for purposes of illustration and not limitation) as a mixerthat receives the first and second representative signals and produces aquasi-static signal proportional to a quasi-static phase differencebetween the first and second representative signals. In an alternateembodiment, phase detector 580 may be implemented with an AND gatehaving as inputs the first and second representative signals and whoseoutput is provided to an integrator. The output of the integrator is aquasi-static signal proportional to a quasi-static phase differencebetween the first representative signal and the second representativesignal. The term “quasi-static” in this context means varying on a timescale substantially similar to a variation in phase, not necessarily atime scale or period substantially similar to that of electromagneticsignal 515. In other embodiments, phase detector 580 may receive orcapture a time domain signal and detect zero crossings or othercharacteristics of wave shape in order to determine an effective phasedifference between the first representative signal and the secondrepresentative second signal. Suitable phase detectors are readilyavailable—such as, by way of example and not by way of limitation, anAnalog Devices part no. AD 8302. Another embodiment of phase detector580 may take digital information from first (H-field) receiver 525 andsecond (E-field) receiver 527 and calculate a phase difference betweenthe first digital information and the second digital information.

[0119] Range detector 590 may be embodied in any means capable ofconverting a measured phase difference to a range r. In a particularsimple example, range detector 590 may be an analog voltmeter having ascale calibrated to read a range r as a function of an applied voltagefrom phase detector 580. A more sophisticated embodiment of rangedetector 590 may, for example, advantageously employ an analog todigital converter and a micro-controller or micro-processor to calculatea range r from an applied voltage received from phase detector 580.Range detector 590 may include visual, audio, or other outputs toindicate range r to a user, or may convey a measured range r to a remotelocation for further analysis as part of a comprehensive tracking,positioning, or locating system.

[0120] Locator 520 may be generally regarded as comprising a means fordetecting and receiving a first signal, a means for detecting andreceiving a second signal, a means for determining a difference betweena first and a second representative signal related to the first andsecond signals and a means for determining a range given a differencebetween the first and second representative signals.

[0121] Beacon 510 may be generally regarded as comprising a means fortransmitting an electromagnetic signal. Beacon 510 may be a fixedreference with respect to which a mobile locator 520 determines adistance or range r. Alternatively, a fixed locator 520 may measurerange r of a mobile beacon 510, or a locator 520 may be a mobile unitthat measures range r of a mobile beacon 510. Furthermore, beacon 510may be an uncooperative transmitter or other source of anelectromagnetic signal 515 whose range r one desires to know withrespect to the position of a locator 520.

[0122] A Preferred Embodiment

[0123]FIG. 6 is a schematic diagram of details of a preferred embodimentof a system for near-field ranging by comparison of electric andmagnetic field phase. In FIG. 6, a ranging system 600 includes a beacon610 and a locator 620 separated from beacon 610 by a range r. Beacon 610includes a transmitter 612 which may be mobile or fixed, and a transmitantenna 636. Transmitter 612 may include means to change properties of atransmitted electromagnetic signal 615 including, by way of illustrationand not by way of limitation, changing frequency, phase, polarization,or amplitude of an electromagnetic signal 615 according to apredetermined pattern, in response to an input or stimulus, such as, forexample, a control signal received from a data bus 695. In alternateembodiments, transmitter 612 may modulate a transmitted electromagneticsignal 615 so as to convey information. Such information may includeinformation that identifies beacon 610 or other information or telemetryof value to a user. For example, binary phase shift keying may beimplemented on a transmitted electromagnetic signal 615 withoutimpairing ranging performance of the present invention. In still anotherembodiment, transmitter 612 may turn on or off according to apredetermined pattern, in response to a control signal from a data bus695, or in response to some other input or stimulus. Such input orstimulus may include (but is not necessarily limited to) a signal froman accelerometer, a timer, a motion detector, other transducers or othersensors.

[0124] It may be advantageous in some applications for transmitter 612to operate at a higher instantaneous power and a lower duty cycle. Forinstance, transmitter 612 might operate at approximately ten times anallowed average power level but only transmit 10% of a characteristicperiod, thus maintaining a substantially similar average transmit powerlevel. Such intermittent operation would enable a higher signal to noiseratio (SNR) signal. Periodic operation of beacon 610 is alsoadvantageous for operation in the presence of interference. When beacon610 is silent (i.e., not transmitting), locator 620 can characterize aparticular coherent noise source such as an interfering signal and cancompensate for the presence of this coherent noise once beacon 610begins transmitting again.

[0125] In applications where security is particularly important, beacon610 may employ techniques to make electromagnetic signal 615 moredifficult to detect by an eavesdropper. These techniques may include afrequency hopping scheme, periodic operation, varying transmit power touse the minimum power needed to make an accurate measurement, or othermeans to render signal 615 less detectable by an eavesdropper. Transmitpower control may be further advantageous to allow frequency reuse insmaller cell sizes.

[0126] A first step in determining range r between beacon 610 andlocator 620 is for a beacon 610 to transmit an electromagnetic signal615. In a preferred embodiment, electromagnetic signal 615 is verticallypolarized, but horizontal polarization or alternate polarizations areusable as well. To avoid unnecessary complication the electromagneticcoupling between beacon 610 and locator 620 is described in terms of anelectromagnetic wave comprising electromagnetic signal 615. Becauserange r between beacon 610 and locator 620 is typically less than awavelength of electromagnetic signal 615, electromagnetic signal 615 isnot typically a radiation electromagnetic wave decoupled from beacon 610such as would be found in the far-field at a range r significantlygreater than one wavelength of electromagnetic signal 615. It should beunderstood that an electromagnetic wave comprising electromagneticsignal 615 is typically a reactive or coupled electromagnetic wave,rather than a radiation or decoupled electromagnetic wave.

[0127] Locator 620 receives electromagnetic signal 615. In a preferredembodiment, locator 620 includes a first (H-field) channel 625, a second(H-field) channel 626, a third (E-field) channel 627, a local oscillator650, a first phase detector 681, a second phase detector 682, and arange detector 690 (including an analog to digital (A/D) converter 691,and a microprocessor 692). An optional data bus 695 may be used toprovide a means for exchanging control and data signals among aplurality of beacons and locators (not shown in detail in FIG. 6).

[0128] First (H-field) channel 625 includes a first (H-field) antenna630, a first (H-field) pre-select filter 6400, a first (H-field) mixer6420, a first (H-field) primary IF filter 6430, a first (H-field)primary IF amplifier 6440, a first (H-field) secondary IF filter 6450, afirst (H-field) secondary IF amplifier 6460, and a first (H-field)automatic gain control 6480. First (H-field) channel 625 has a first(H-field) antenna port 6270, a first (H-field) tuning port 6230, a first(H-field) received signal strength indicator (RSSI) port 6220, and afirst (H-field) signal output port 6210.

[0129] A second (H-field) channel 626 includes a second (H-field)antenna 631, a second (H-field) pre-select filter 6401, a second(H-field) mixer 6421, a second (H-field) primary IF filter 6431, asecond (H-field) primary IF amplifier 6441, a second (H-field) secondaryIF filter 6451, a second (H-field) secondary IF amplifier 6461, and asecond (H-field) automatic gain control 6481. Second (H-field) channel626 has a second (H-field) antenna port 6271, a second (H-field) tuningport 6231, a second (H-field) received signal strength indicator (RSSI)port 6221, and a second (H-field) signal output port 6211.

[0130] A third (E-field) channel 627 includes a third (E-field) antenna632, a third (E-field) pre-select filter 6402, a third (E-field) mixer6422, a third (E-field) primary IF filter 6432, a third (E-field)primary IF amplifier 6442, a third (E-field) secondary IF filter 6452, athird (E-field) secondary IF amplifier 6462, and a third (E-field)automatic gain control 6482. Third (E-field) channel 627 has a third(E-field) antenna port 6272, a third (E-field) tuning port 6232, a third(E-field) received signal strength indicator (RSSI) port 6222, and athird (E-field) signal output port 6212.

[0131] First (H-field) antenna 630 is responsive to the magnetic orH-field component of electromagnetic signal 615 and presents a receivedsignal proportional to the magnetic or H-field component ofelectromagnetic signal 615 to first (H-field) pre-select filter 6400.

[0132] First (H-field) pre-select filter 6400 passes a firstrepresentative signal proportional to the magnetic or H-field componentof electromagnetic signal 615, but rejects signals with undesirablefrequencies. First (H-field) pre-select filter 6400 may be, for example,a band pass filter or a low pass filter. Typically first (H-field)pre-select filter 6400 will pass those frequencies within which beacon610 might transmit an electromagnetic signal 615 for a relevantapplication. Selection of a band will depend upon a variety of factorsincluding, but not necessarily limited to, regulatory constraints,propagation behavior of electromagnetic signal 615, and a desired ranger of operation.

[0133] First (H-field) mixer 6420 mixes the first representative signalreceived from first (H-field) pre-select filter 6400 with a localoscillator (LO) signal generated by local oscillator 650 to generate afirst intermediate frequency (or IF) representative signal. Localoscillator 650 may be a traditional sine wave oscillator, a directdigital synthesizer (DDS), or other oscillator or waveform templatesource.

[0134] First primary (H-field) IF filter 6430 accepts only the desiredfirst IF representative signal and rejects other undesired signals. Acrystal filter may be advantageously used as first primary (H-field) IFfilter 6430. Such a crystal filter is characterized by an extremelynarrow pass band, and preferably has a constant group delay within thepass band. A narrow pass band acts to allow the desired first IFrepresentative signal to be conveyed to first primary (H-field) IFamplifier 6440 while rejecting adjacent undesired signals. First primary(H-field) IF amplifier 6440 increases the amplitude of the first IFrepresentative signal and conveys the amplified first IF representativesignal to first secondary (H-field) IF filter 6450. First secondary(H-field) IF filter 6450 accepts only the desired first IFrepresentative signal and rejects other undesired signals. A crystalfilter may be advantageously used as first secondary (H-field) IF filter6450. Such a crystal filter is characterized by an extremely narrow passband, and preferably has a constant group delay within the pass band. Anarrow pass band acts so as to allow the desired first IF representativesignal to be conveyed to first secondary (H-field) IF amplifier 6460while rejecting adjacent undesired signals. First secondary (H-field) IFamplifier 6460 increases the amplitude of the first IF representativesignal and conveys the first IF representative signal to signal outputport 6210 and to first automatic gain control (AGC) 6480.

[0135] First automatic gain control 6480 adjusts a gain of first primary(H-field) IF amplifier 6440 and first secondary (H-field) IF amplifier6460 to maintain a desired level of the first IF representative signal.By dividing a desired total gain between first primary (H-field) IFamplifier 6440 and first secondary (H-field) IF amplifier 6460, a hightotal gain and a large dynamic range can be maintained with greaterstability and reliability than in a single amplification stage alone.Similarly, by dividing the desired filtering between first primary(H-field) IF filter 6430 and first secondary (H-field) IF filter 6450, amore narrow passband can be achieved with greater stability and greaterreliability than with a single filter stage alone. First automatic gaincontrol 6480 preferably includes a received signal strength indicator(RSSI) and conveys an RSSI level to RSSI output 6220.

[0136] Second (H-field) antenna 631 is responsive to the magnetic orH-field component of electromagnetic signal 615 and presents a receivedsignal proportional to the magnetic or H-field component ofelectromagnetic signal 615 to second (H-field) pre-select filter 6401.

[0137] Second (H-field) pre-select filter 6401 passes a firstrepresentative signal proportional to the magnetic or H-field componentof electromagnetic signal 615, but rejects signals with undesirablefrequencies. Second (H-field) pre-select filter 6401 may be, forexample, a band pass filter or a low pass filter. Typically second(H-field) pre-select filter 6401 will pass those frequencies withinwhich beacon 610 might transmit an electromagnetic signal 615 for arelevant application. Selection of a band will depend upon a variety offactors including, but not necessarily limited to, regulatoryconstraints, propagation behavior of electromagnetic signal 615, and adesired range r of operation.

[0138] Second (H-field) mixer 6421 mixes the first representative signalreceived from second (H-field) pre-select filter 6401 with a localoscillator (LO) signal generated by local oscillator 650 to generate asecond intermediate frequency (or IF) representative signal. Localoscillator 650 may be a traditional sine wave oscillator, a directdigital synthesizer (DDS), or other oscillator or waveform templatesource.

[0139] Second primary (H-field) IF filter 6431 accepts only the desiredsecond IF representative signal and rejects other undesired signals. Acrystal filter may be advantageously used as second primary (H-field) IFfilter 6431. Such a crystal filter is characterized by an extremelynarrow pass band, and preferably has a constant group delay within thepass band. A narrow pass band acts to allow the desired second IFrepresentative signal to be conveyed to second primary (H-field) IFamplifier 6441 while rejecting adjacent undesired signals. Secondprimary (H-field) IF amplifier 6441 increases the amplitude of thesecond IF representative signal and conveys the amplified second IFrepresentative signal to second secondary (H-field) IF filter 6451.Second secondary (H-field) IF filter 6451 accepts only the desiredsecond IF representative signal and rejects other undesired signals. Acrystal filter may be advantageously used as second secondary (H-field)IF filter 6451. Such a crystal filter is characterized by an extremelynarrow pass band, and preferably has a constant group delay within thepass band. A narrow pass band acts so as to allow the desired second IFrepresentative signal to be conveyed to second secondary (H-field) IFamplifier 6461 while rejecting adjacent undesired signals. Secondsecondary (H-field) IF amplifier 6461 increases the amplitude of thesecond IF representative signal and conveys the second IF representativesignal to signal output port 6211 and to second automatic gain control(AGC) 6481.

[0140] Second automatic gain control 6481 adjusts a gain of secondprimary (H-field) IF amplifier 6441 and second secondary (H-field) IFamplifier 6461 to maintain a desired level of the second IFrepresentative signal. By dividing a desired total gain between secondprimary (H-field) IF amplifier 6441 and second secondary (H-field) IFamplifier 6461, a high total gain and a large dynamic range can bemaintained with greater stability and reliability than in a singleamplification stage alone. Similarly, by dividing the desired filteringbetween second primary (H-field) IF filter 6431 and second secondary(H-field) IF filter 6451, a narrower passband can be achieved withgreater stability and greater reliability than with a single filterstage alone. Second automatic gain control 6481 preferably includes areceived signal strength indicator (RSSI) and conveys an RSSI level toRSSI output 6221.

[0141] Third (E-field) antenna 632 is responsive to the electric orE-field component of electromagnetic signal 615 and presents a receivedsignal proportional to the electric or E-field component ofelectromagnetic signal 615 to third (E-field) pre-select filter 6402.

[0142] Third (E-field) pre-select filter 6402 passes a thirdrepresentative signal proportional to the electric or E-field componentof electromagnetic signal 615, but rejects signals with undesirablefrequencies. Third (E-field) pre-select filter 6402 may be, for example,a band pass filter or a low pass filter. Typically third (E-field)pre-select filter 6402 will pass those frequencies within which beacon610 might transmit an electromagnetic signal 615 for a relevantapplication. Selection of a band will depend upon a variety of factorsincluding, but not necessarily limited to, regulatory constraints,propagation behavior of electromagnetic signal 615, and a desired ranger of operation.

[0143] Third (E-field) mixer 6422 mixes the third representative signalreceived from third (E-field) pre-select filter 6402 with a localoscillator (LO) signal generated by local oscillator 650 to generate athird intermediate frequency (or IF) representative signal. Localoscillator 650 may be a traditional sine wave oscillator, a directdigital synthesizer (DDS), or other oscillator or waveform templatesource.

[0144] Third primary (E-field) IF filter 6432 accepts only the desiredthird IF representative signal and rejects other undesired signals. Acrystal filter may be advantageously used as third primary (E-field) IFfilter 6432. Such a crystal filter is characterized by an extremelynarrow pass band, and preferably has a constant group delay within thepass band. A narrow pass band acts to allow the desired third IFrepresentative signal to be conveyed to third primary (E-field) IFamplifier 6442 while rejecting adjacent undesired signals. Third primary(E-field) IF amplifier 6442 increases the amplitude of the third IFrepresentative signal and conveys the amplified third IF representativesignal to third secondary (E-field) IF filter 6452. Third secondary(E-field) IF filter 6452 accepts only the desired third IFrepresentative signal and rejects other undesired signals A crystalfilter may be advantageously used as third secondary (E-field) IF filter6452. Such a crystal filter is characterized by an extremely narrow passband, and preferably has a constant group delay within the pass band. Anarrow pass band acts so as to allow the desired third IF representativesignal to be conveyed to third secondary (E-field) IF amplifier 6462while rejecting adjacent undesired signals. Third secondary (E-field) IFamplifier 6462 increases the amplitude of the third IF representativesignal and conveys the third IF representative signal to signal outputport 6212 and to third automatic gain control (AGC) 6482.

[0145] Third automatic gain control 6482 adjusts a gain of secondprimary (E-field) IF amplifier 6442 and third secondary (E-field) IFamplifier 6462 to maintain a desired level of the third IFrepresentative signal. By dividing a desired total gain between thirdprimary (E-field) IF amplifier 6442 and third secondary (E-field) IFamplifier 6462, a high total gain and a large dynamic range can bemaintained with greater stability and reliability than in a singleamplification stage alone. Similarly, by dividing the desired filteringbetween third primary (E-field) IF filter 6432 and third secondary(E-field) IF filter 6452, a more narrow passband can be achieved withgreater stability and greater reliability than with a single filterstage alone. Third automatic gain control 6482 preferably includes areceived signal strength indicator (RSSI) and conveys an RSSI level toRSSI output 6222.

[0146] Local oscillator 650 may also be advantageously used as a tunerto select among a plurality of electromagnetic signals 615 transmittedby a plurality of beacons 610. A particular beacon 610 emitting aparticular electromagnetic signal 615 may be distinguished from otherbeacons 610 emitting other electromagnetic signals 615 with slightlydifferent frequencies. Thus a single locator 620 may track a largenumber of different beacons 610. A variety of other schemes for trackingmultiple beacons 610 are possible, including, for example, time divisionmultiple access. If a beacon 610 modulates a transmitted electromagneticsignal 615 with identifying information, one can distinguish among aplurality of beacons 610 operating at the same frequency. Similarly, alarge number of different locators 620 may measure ranges r to a commonbeacon 610.

[0147] Although synchronization is not required between beacon 610 andlocator 620, a common local oscillator 650 acts to maintainsynchronization among a plurality of channels 625, 626, 627 within asingle locator 620. Synchronization among plurality of channels 625,626, 627 within locator 620 is advantageous to enable precision phasecomparisons among signals received by plurality of channels 625, 626,627.

[0148] In other embodiments, local oscillator 650 may tune a firstchannel 625, a second channel 626, or a third channel 627 (or variouscombinations of channels 625, 626, 627) to sweep through a variety offrequencies of interest. Micro-processor 692 may monitor and compiledata from RSSI ports 6220, 6221, 6222 (or various combinations of RSSIports 6220, 6221, 6222) to characterize a noise and interferenceenvironment. Micro-processor 692 may convey appropriate control signalsthrough data bus 695 to a plurality of beacons 610 to select optimalfrequencies or modes of operation given a characterized noise andinterference environment. Similarly, in a dense signal environment withmany simultaneously operating beacons 610, micro-processor 692 maymonitor signals and convey appropriate control signals through data bus695 to a plurality of beacons 610 to assign optimal frequencies or modesof operation among a plurality of beacons 610 for facilitatingcoexistence within and among the plurality of beacons 610. Further,micro-processor 692 may monitor range r and convey appropriate controlsignals through data bus 695 to a respective beacon 610 to assign anoptimal frequency or mode of operation appropriate for the respectivebeacon 610 appropriate for operation at a detected range r to therespective beacon 610.

[0149] In other embodiments, channels in addition to channels 625, 626,627 may be used so that a locator 620 may simultaneously track aplurality of beacons 610 generating electromagnetic signals 615 atdifferent frequencies. Further, additional channels may beadvantageously employed in detecting and characterizing a noise andinterference environment. In still other embodiments, additionalchannels associated with alternate polarizations may enable rangingsystem 600 to make measurements unimpaired by the relative orientationof a beacon 610 with respect to a locator 620.

[0150] In ranging system 600 (FIG. 6), first phase detector 681 receivesthe first IF representative signal from first signal output port 6210and receives the third IF representative signal from third signal outputport 6212 and determines phase difference between the first and third IFrepresentative signals. Second phase detector 682 receives the second IFrepresentative signal from second signal output port 6211 and receivesthe third IF representative signal from third signal output port 6212and determines phase difference between the second and third IFrepresentative signals. In a preferred embodiment, locator 620 has twoH-field channels (first (H-field) channel 625 and second (H-field)channel 626) and a third (E-field) channel 627. In a preferredembodiment using a vertically polarized electromagnetic signal 615,third electric antenna 632 is a vertical whip antenna with anomni-directional pattern in a first plane perpendicular to the axis ofthe whip. In a preferred embodiment magnetic antennas 630, 631 are loopantennas with an omni-directional pattern in a second planesubstantially perpendicular to a first plane (associated with the whipantenna of third electric antenna 632). It is advantageous to have twomagnetic antennas 630, 631 to achieve sensitivity to a magneticcomponent of an electromagnetic signal 615 incident in any direction.With only one magnetic antenna 630 or 631 locator 620 will tend beinsensitive to a beacon 610 positioned in a direction that lies in anull of the single magnetic antenna 630 or 631. By having two magneticantennas 630, 631 locator 620 can determine range r to a beacon 610 inany direction. An additional advantage of having two magnetic antennas630, 631 is that locator 620 may use prior art techniques to obtainangle of arrival information in addition to range information.

[0151] For optimal performance of phase detectors 681, 682 it isadvantageous for amplitudes of first, second and third IF representativesignals to be maintained within a desired amplitude limit. Automaticgain controls 6480, 6481, 6482 act to maintain a desired amplitude limitfor the first, second and third IF representative signals. Phasedetectors 681, 682 may employ log amps to maintain constant signallevels, such as are used in an Analog Devices part no. AD 8302 (phasedetector IC). Alternatively, channels 625, 626, 627 may include alimiter (not shown in FIG. 6) to limit output signal levels.

[0152] Range detector 690 translates measured phase differences receivedfrom phase detectors 681, 682 to range r. In a preferred embodiment,range detector 690 includes an analog to digital converter 691 and amicroprocessor (or a micro-controller) 692 that cooperate to calculaterange r based upon signals received from one or both of phase detectors681, 682. In a preferred embodiment, range detector 690 also monitorsRSSI levels from RSSI ports 6220, 6221, 6222 so that range detector 690can select either of phase detectors 681, 682 (or both) to use indetermining range r. Range detector 690 may also compare RSSI levelsfrom RSSI ports 6220, 6221, 6222 to determine angle of arrival ofelectromagnetic signal 615. Typically first phase detector 681 will bepreferred if beacon 610 lies in the pattern of first magnetic fieldantenna 630 and second phase detector 682 will be preferred if beacon610 lies in the pattern of second magnetic field antenna 631. Ideallyrange detector 690 will selectively employ signals received from phasedetectors 682, 682 to optimize range measurement. Such optimizationmight also involve, for example, locator 620 combining signals receivedfrom magnetic field antennas 630, 631 to create an effective antennapattern that nulls out an interfering signal, or maximizes a desiredsignal. RSSI levels from RSSI ports 6220, 6221, 6222 may also be used byrange detector 690 to supplement or complement information from phasedetectors 681, 682 in determining range r.

[0153] Range detector 690 may include visual, audio, or other outputformats to indicate range r to a user, or may convey a measured range rto a remote location for further analysis as part of a comprehensivepositioning, tracking, or locating system. Range detector 690 may alsoinclude means to control local oscillator 650 including (but notnecessarily limited to) setting a frequency of a local oscillatorsignal.

[0154] Data bus 695 is optional and when employed allows data andcontrol signals to be conveyed between locator 620 and beacon 610. Databus 695 may involve a wireless network (such as an 802.11b network), ahard wired network (such as an Ethernet connection or a serial cable),or may employ modulation of electromagnetic signal 615 transmitted bybeacon 610. A plurality of locators 620 and beacons 610 may share acommon data bus 695. Such a plurality of locators 620 and beacons 610may operate cooperatively to establish a comprehensive tracking,positioning, or locating system. With a wireless data bus 695, beacon610 is no longer strictly a transmit-only device. Because only atransmitted electromagnetic signal 615 is necessary for rangingoperations, with a wireless data link precise timing required for atraditional transponder ranging system is eliminated. Timing informationcan be conveyed via the wireless data link.

[0155] Locator 620 may be regarded as comprising a means for detectingor receiving a first (H-field) signal, a means for detecting orreceiving a second (H-field) signal, a means for detecting or receivinga third (E-field) signal, a means for determining a first phasedifference between a first and a third signal, a means for determining asecond phase difference between a second and a third signal, and a meansfor determining a range r given a first and a second phase difference.It may also be advantageous to include in locator 620 a means for tuninga locator 620 whereby range data may be obtained for any of a pluralityof beacons 610, each generating an electromagnetic signal at a differentfrequency.

[0156] Still further advantages may accrue by adding to locator 620 ameans for conveying data among a plurality of locators 620 and aplurality of beacons. Such a means (e.g., a data bus or a wireless link695) could be advantageously employed in a comprehensive tracking,positioning, or locating system.

[0157] It should be kept in mind that functions and components oflocator 620 need not be implemented in a single unit. For example, itmay be advantageous to place first (H-field) antenna 630, second(H-field) antenna 631, and third (E-field) antenna 632 at respectivelocations distant from other components or functionality of locator 620.Antennas may, for example, be connected via RF cables if a stand-offwere desired for safety reasons, economic reasons, operational reasons,ease-of-use or for any other reasons. Similarly, locator 620 mayimplement signal detection and reception in one location and phasedetection in another. Locator 620 may also implement phase detection inone location and relay data to a range detector 690 at a remote locationfor determination of range r.

[0158] Combined Beacon-Locator

[0159]FIG. 7 is a schematic diagram of a system for near-field rangingby comparison of electric and magnetic field phase with beacon andlocator function combined in a single unitary device. In FIG. 7, acombined beacon-locator apparatus 700 is configured to operate as abeacon whose range r from a remote locator (such as a remotebeacon-locator apparatus 710 operating as a locator) may be measured bythe remote locator. Alternatively, beacon-locator apparatus 700 canoperate as a locator that measures range r to another beacon (such asremote beacon-locator apparatus 710 operating as a beacon).

[0160] Beacon-locator apparatus 700 includes a first magnetic (H-field)antenna 730, a second (E-field) antenna 732, a transmit-receive switch728, a transmitter 712, and a locator receiver 720. Locator receiver 720includes a first (H-field) receiver 722, a second (E-field) receiver742, a phase detector 781, and a range detector 790. An optional databus 795 permits communication between or among a plurality ofbeacon-locators, beacons, locators, or other devices.

[0161] Combined Beacon-Locator in Locator Mode

[0162] Remote beacon-locator apparatus 710 (operating in a bacon mode)transmits an electromagnetic signal 715 that is received bybeacon-locator system 700 operating in a locator mode. First (H-field)antenna 730 is sensitive to a magnetic component of an incidentelectromagnetic signal 715 and conveys a representative magnetic signalproportional to the magnetic component of electromagnetic signal 715 toan antenna port 7270 of first (H-field) receiver 722.

[0163] First (H-field) receiver 722 receives the representative magneticsignal at first antenna port 7220, and receives a local oscillator (LO)signal from a local oscillator 750 at a local oscillator port 7230.Using filtering, amplification and mixing means generally known topractitioners of the RF arts (an example of which is described inconnection with FIG. 6), first (H-field) receiver 722 presents a firstreceived intermediate frequency (IF) representative signal at a firstoutput port 7210 and an RSSI signal at an RSSI port 7220.

[0164] Because beacon-locator apparatus 700 is operating in a locatormode, transmit-receive switch 728 is set to couple second (E-field)antenna 732 to second (E-field) receiver 742. In an alternateembodiment, transmit-receive switch 728 may be a circulator or otherdevice that allows a beacon-locator, such as beacon-locator apparatus700, to function as a beacon and as a locator simultaneously. Second(E-field) antenna 732 sensitive to the electric component of incidentelectromagnetic signal 715 and conveys a representative electric signalproportional to the electric component of electromagnetic signal 715 toan antenna port 7271 of second (E-field) receiver 742.

[0165] Second (E-field) receiver 742 receives the representativeelectric signal at second antenna port 7271, and receives a localoscillator (LO) signal from local oscillator 750 at a local oscillatorport 7231. Using filtering, amplification and mixing means generallyknown to practitioners of the RF arts (an example of which is discussedin connection with FIG. 6), second (E-field) receiver 742 presents asecond received intermediate frequency (IF) representative signal at asecond output port 7211 and an RSSI signal at an RSSI port 7221.

[0166] Phase detector 781 receives the first representative signal fromoutput port 7210 and receives the second representative signal fromoutput port 7211. Phase detector 781 generates a phase difference outputsignal proportional to the phase difference between the first and secondrepresentative signals and conveys the phase difference output signal torange detector 790.

[0167] Range detector 790 includes an analog to digital converter 791and a micro-processor 792. Range detector 790 receives RSSI signals fromRSSI ports 7220, 7221 and the phase difference output signal from aphase detector 781. Analog to digital converter 791 converts thesesignals to digital signals and conveys them to micro-processor 792.Micro-processor 792 calculates range r based upon the digital signalinputs received from analog to digital converter 791. Among the means bywhich a micro-processor 792 may determine a range r are, for example: 1)Free space theory as presented in Equation [8], 2) a more preciseanalytical or numerical model including ground and other effects of apropagation environment, and 3) a model based upon empiricalmeasurements. Range r may be calculated from a phase input alone orusing a more complicated model including input from RSSI ports 7220,7221.

[0168] Micro-processor 792 may adjust a frequency of local oscillator750 to tune first (H-field) receiver 722 and second (E-field) receiver742. This enables beacon-locator apparatus 700 to measure range r of avariety of other beacons 710 or beacon-locators 700 operating atdifferent frequencies. Micro-processor 792 also enables beacon-locatorapparatus 700 to use a frequency hopping system or power control schemefor added security and robustness.

[0169] Micro-processor 792 may have a user interface means such as anaudio or visual display to provide a user with a range measurement. Inaddition micro-processor 792 may convey range or other information toanother location via an optional data bus 795 as part of a comprehensivesystem that relies on tracking or positioning input, or for anotherpurpose.

[0170] Exemplary beacon-locator apparatus 700 has two channels, first(H-field) receiver channel 722 and second (E-field) receiver channel742. Additional channels may be preferred if better performance isdesired at the cost of additional complexity and expense. Suchadditional channels could be used to detect E-field and H-fieldcomponents in alternate polarizations including but not limited topolarization components longitudinal to a direction of an incidentelectromagnetic signal 715. Thus beacon-locator apparatus 700 could beless dependent upon a particular orientation of an incidentelectromagnetic signal 715 and thereby offer more robust performance.These same benefits also accrue for locators that are not combined withbeacons to form beacon-locators.

[0171] Combined Beacon-Locator in Beacon Mode

[0172] When beacon-locator apparatus 700 operates in a beacon mode,micro-processor 792 triggers transmit-receive switch 728 to connecttransmitter 712 to antenna 732. Micro-processor 792 also sets anappropriate frequency for a transmitter 712. Exemplary beacon-locatorapparatus 700 uses electric antenna 732 as a beacon transmit antenna.Magnetic antenna 730 could just as readily be used. The choice ofantenna to be used for transmission operation in a beacon mode dependsupon several factors including, for example, pattern, performance inproximity of other objects, polarization, matching, and propagationenvironment.

[0173] Remote beacon-locator apparatus 710 includes an electric antenna735 and a magnetic antenna 733. Transmitter 712 sends an RF signal totransmit antenna 732. Transmit antenna 732 radiates an electromagneticsignal 716 that is received by electric antenna 735 and by magneticantenna 733 when remote beacon-locator apparatus 710 operates in alocator mode. Remote beacon-locator apparatus 710 receives an H-fieldsignal from magnetic antenna 733 and receives an E-field signal fromelectric antenna 735 thus allowing remote beacon-locator apparatus 710to determine range r to beacon-locator apparatus 700.

[0174] An optional data bus 795 allows beacon-locator apparatus 700 tointeract and coordinate with remote beacon-locator apparatus 710. Forexample, beacon-locator apparatus 700 can trigger remote beacon-locatorapparatus 710 to cause remote beacon-locator apparatus 710 to transmitand allow beacon-locator apparatus 700 to determine range r to remotebeacon-locator apparatus 710. An appropriate trigger might, for example,include data regarding a communication frequency, a frequency-hoppingpattern, power control feedback or another characteristics of a transmitsignal to be radiated from remote beacon-locator apparatus 710. Atrigger might further include identification or authenticationinformation.

[0175] Transmitter 712 may be controlled by micro-processor 792 tomodulate electromagnetic signal 716 with information. A wide variety ofmodulation techniques are possible. Binary phase shift key (BPSK) is onepreferred modulation option. BPSK is advantageous because of itssimplicity. Further, because the present invention relies on a relativedifference between electric and magnetic field phases, a common modephase shift (such as happens with BPSK and similar modulations) does noteffect the ability of the present invention to measure range r. Suchinformation may include identifying or authentication information, orother information or telemetry of value to a user.

[0176] Antenna Configurations

[0177] FIGS. 8-11 reveal a variety of antenna configurations for rangingsystems 800, 900, 1000, 1100. FIG. 8 is a schematic illustration of arepresentative antenna configuration for a near-field ranging systemhaving a vertical polarization beacon and a vertical polarizationomni-directional locator. In FIG. 8, ranging system 800 includes avertical polarization beacon 810 and locator 820. A verticalpolarization antenna 836 associated with vertical polarization beacon810 is typically a vertically oriented whip or dipole antenna, but couldbe a loop or loopstick antenna oriented to radiate vertically polarizedelectromagnetic signals 815 in a desired direction. In many applicationsan omni-directional coverage of a single vertically oriented whip ispreferred to a more directional pattern of a traditional verticallypolarized loop. Locator 820 includes an electric antenna 832, a firstmagnetic antenna 831, and a second magnetic antenna 833 orientedperpendicularly to first magnetic antenna 831. Electric antenna 832 istypically a vertically oriented whip or dipole antenna. First magneticantenna 831 and second magnetic antenna 833 are typically loop orloopstick antennas oriented to be responsive to vertically polarizedelectromagnetic signal 815. Locator 820 can select either first magneticantenna 831 or second magnetic antenna 833 to optimize a received(H-field) signal. Locator 820 may also use signals from both firstmagnetic antenna 831 and second magnetic antenna 833.

[0178]FIG. 9 is a schematic illustration of a representative antennaconfiguration for a near-field ranging system having a horizontalpolarization beacon and a horizontal polarization omni-directionallocator. In FIG. 9, ranging system 900 includes a horizontalpolarization beacon 910 and locator 920. A horizontal polarizationantenna 937 associated with horizontal polarization beacon 910 istypically a vertically oriented loopstick or loop antenna oriented in ahorizontal plane, but could be a whip or dipole antenna oriented toradiate horizontally polarized electromagnetic signals 915 in a desireddirection. In many applications the omni-directional coverage of asingle loop or loopstick antenna is preferred to a more directionalpattern of a traditional horizontally polarized whip or dipole antenna.Locator 920 includes a magnetic antenna 931, a first electric antenna932, and a second electric antenna 934. Magnetic antenna 931 istypically a vertically oriented loopstick or loop antenna oriented in ahorizontal plane. First electric antenna 932 and second electric antenna934 are typically dipole or whip antennas oriented to be responsive tohorizontally polarized electromagnetic signals 915. Locator 920 canselect either first electric antenna 932 or second electric antenna 934to optimize a received (E-field) signal. Locator 920 may also usesignals from both first electric antenna 932 and second electric antenna934.

[0179]FIG. 10 is a schematic illustration of a representative antennaconfiguration for a near-field ranging system having a verticalpolarization beacon and a vertical polarization directional locator. InFIG. 10, ranging system 1000 includes a vertical polarization beacon1010 and locator 1020. A vertical polarization antenna 1036 associatedwith vertical polarization beacon 1010 is typically a verticallyoriented whip or dipole antenna oriented in a vertical plane, but couldbe a loop or loopstick antenna oriented to radiate vertically polarizedelectromagnetic signals 1015 in a desired direction. In manyapplications the omni-directional coverage of a single verticallyoriented whip antenna is preferred to a more directional pattern of atraditional vertically polarized loop antenna. Locator 1020 includes anelectric antenna 1032 and a magnetic antenna 1031. Electric antenna 1032is typically a vertically oriented whip or dipole antenna. Magneticantenna 1031 is typically a loop or loopstick antenna oriented to beresponsive to vertically polarized electromagnetic signals 1015. Locator1020 typically must be oriented to optimize a signal from magneticantenna 1031. Additionally, the direction of arrival of electromagneticsignal 1015 can be determined by orienting a null of magnetic antenna1031 with the direction of arrival of electromagnetic signal 1015 andobserving an associated decrease in an RSSI level. If the responses ofmagnetic antenna 1031 and electric antenna 1032 are summed, thedirection of arrival of electromagnetic signal 1015 can be determined byorienting a null of an effective summed pattern with a direction ofarrival of electromagnetic signal 1015 and observing an associateddecrease in amplitude of the summed responses.

[0180]FIG. 11 is a schematic illustration of a representative antennaconfiguration for a near-field ranging system having a horizontalpolarization beacon and a horizontal polarization directional locator.In FIG. 11, ranging system 1100 includes a horizontal polarizationbeacon 1110 and locator 1120. A horizontal polarization antenna 1137associated with horizontal polarization beacon 1110 is typically aloopstick antenna oriented vertically or a loop antenna oriented in ahorizontal plane, but could be a whip or dipole antenna oriented toradiate horizontally polarized electromagnetic signals 1115 in a desireddirection. In many applications the omni-directional coverage of asingle loop or loopstick antenna is preferred to a more directionalpattern of a traditional horizontally polarized whip or dipole antenna.Locator 1120 includes an electric antenna 1132 and a magnetic antenna1131. Electric antenna 1132 is typically a horizontally oriented whip ordipole antenna. Magnetic antenna 1131 is typically a loop or loopstickantenna oriented to be responsive to horizontally polarizedelectromagnetic signals 1115. Locator 1120 typically must be oriented tooptimize a signal from electric antenna 1132. Additionally, thedirection of arrival of electromagnetic signal 1115 can be determined byorienting a null of electric antenna 1132 with the direction of arrivalof electromagnetic signal 1115 and observing an associated decrease inan RSSI level. If the responses of magnetic antenna 1131 and electricantenna 1132 are summed, the direction of arrival of electromagneticsignal 1115 can be determined by orienting a null of an effective summedpattern with a direction of arrival of electromagnetic signal 1115 andobserving an associated decrease in amplitude of the summed responses.

[0181] A choice of polarization may be influenced by specifics of aparticular propagation environment, by the presence of potentiallyinterfering signals of a particular polarization, or by the requirementsof a particular application. Vertical polarization is typicallypreferred for propagation in an environment where undesired couplingtends to be horizontal, such as near ground. Horizontal polarization istypically preferred for propagation in an environment where undesiredcoupling is vertical such as through vertically oriented steel members.Circular polarization is typically preferred for systems whereorientation independence is important. Some such coupling may actuallybe desirable if this coupling tends to guide waves in a desireddirection.

[0182] Important antenna parameters for designing ranging systemsaccording to the present invention include antenna patterns, matching,form factors, performance and cost. Another important critical parameteris capturing and differentiating between an electric and a magneticcomponent of an incident electromagnetic signal. A wide variety ofsuitable antenna options are known to those skilled in the RF arts.

[0183] Exemplary Receiver

[0184] The inventors have implemented a ranging system as taught by thepresent invention. This system operated at 10.7 MHz and exhibitedranging accuracies within inches from about 5 ft to about 35 ft. Sincethe wavelength (λ) at 10.7 MHz is 92 ft, this corresponds to about 0.054λ to 0.38 λ. According to the teachings of the present invention,significantly longer ranges are possible by utilizing significantlylower frequencies.

[0185]FIG. 12 is a schematic diagram illustrating details of anexemplary receiver in a system for electromagnetic ranging. In FIG. 12,a ranging system 1200 includes a beacon 1210 and a locator 1220. Beacon1210 transmits an electromagnetic signal 1215 that is received bylocator 1220. Locator 1220 includes an electric antenna 1232 that issensitive to the electric component of electromagnetic signal 1215.Electric antenna 1232 detects a first (electric or E-field) signalproportional to the electric component of electromagnetic signal 1215and conveys the first signal to an antenna port 1270 of a first receiver1225 included in locator 1220. Locator 1220 also includes a magneticantenna 1231 that is sensitive to the magnetic component ofelectromagnetic signal 1215. Magnetic antenna 1231 detects a second(magnetic or H-field) signal proportional to the magnetic component ofelectromagnetic signal 1215 and conveys the second signal to a secondreceiver 1227 included in locater 1220. Second receiver 1227 isconstructed in substantial similarity to receiver 1225; details ofconstruction of receiver 1227 are omitted in FIG. 12 in order tosimplify the description of ranging system 1200.

[0186] Exact spacing between electric antenna 1232 and magnetic antenna1231 is not critical, providing that spacing is large enough to avoidundesired mutual coupling and spacing is small relative to thewavelength λ of electromagnetic signal 1215. The inventors have arrangedelectric antenna 1232 and magnetic antenna 1231 separated by a distanceon the order of 1%-3% of a wavelength (0.03 λ-0.01 λ). In alternateembodiments, electric antenna 1232 and magnetic antenna 1231 may bearranged in a single integral unit with a first terminal yielding anE-field response and a second terminal yielding an H-field response.Although spacing between antennas is preferentially small relative tothe wavelength λ of electromagnetic signal 1215, a larger spacingbetween electric antenna 1232 and magnetic antenna 1231 may be toleratedif phase detector 1280 or range detector 1290 in locator 1220 arecompensated for the effect of the larger spacing.

[0187] Locator 1220 also includes a pre-select filter 1242 that receivesthe first (electric) signal from antenna port 1270. Pre-select filter1242 passes the first (electric) signal in a desired band, but rejectssignals with undesirable frequencies. Typically pre-select filter 1242will pass a band of frequencies within which beacon 1210 might transmitan electromagnetic signal 1215 for a relevant application. Selection ofa band will depend upon a variety of factors including, but notnecessarily limited to, regulatory constraints, propagation behavior ofan electromagnetic signal 1215, and a desired range r of operation. Thepresent invention offers optimal performance for a desired range r ofoperation approximately constrained by 0.08 λ to 0.30 λ, where k is thewavelength of the electromagnetic signal 1215 transmitted by beacon1210. A typical operating range is generally within 0.05 λ to 0.50 λ.Higher performance implementations of the present invention may operateat ranges r less than 0.05 λ and greater than 0.50 λ.

[0188] A front-end-amplifier 1265 increases the amplitude of the first(electric) signal. If atmospheric and other noise are sufficiently low,it is advantageous for an amplifier to have a noise figure sufficientlylow to avoid introducing undesired noise, a dynamic range large enoughto accommodate the potential variation in amplitude of the first(electric) signal, and a gain sufficient to yield a suitably largeamplitude first (electric) signal so that a weak signal will properlydrive phase detector 1281. The inventors have advantageously used aMini-Circuits ZFL-500 amplifier as a front-end-amplifier 1265, but awide variety of other amplifiers are suitable.

[0189] A mixer 1252 mixes the first (electric) signal with a localoscillator (LO) signal generated by a local oscillator 1250 thusyielding a first intermediate frequency (IF) signal. Local oscillator1250 may be a traditional sine wave oscillator. Local oscillator 1250may also be a direct digital synthesizer (DDS), or other waveformtemplate generator. For instance, the inventors have used an AnalogDevices DDS (AD 9835) as local oscillator 1250 and a Mini-Circuits SBL-3mixer as mixer 1252. A wide variety of alternate implementations arepossible.

[0190] An IF amplifier 1262 increases the amplitude of the first IFsignal. The inventors have found that a pair of current feedbackoperational amplifiers providing about +50 dB of gain were a suitableembodiment of IF amplifier 1262, but a wide variety of alternatives areavailable to practitioners of the RF arts.

[0191] An IF filter 1244 accepts only the desired first IF signal andrejects other undesired signals. A crystal filter may be advantageouslyused as IF filter 1244. Such a crystal filter is characterized by anextremely narrow pass band, and preferably has a constant group delaywithin the pass band. A narrow pass band acts so as to allow the desiredfirst IF signal to be conveyed to phase detector 1281 while rejectingadjacent undesired signals and noise.

[0192] Local oscillator 1250 may also be advantageously used as a tunerto select among a plurality of electromagnetic signals transmitted by aplurality of beacons 1210. A particular beacon 1210 emitting aparticular electromagnetic signal may be distinguished from otherbeacons emitting other electromagnetic signals, where other signals haveslightly different frequencies. Thus a single locator 1220 may track alarge number of different beacons 1210. A variety of other schemes fortracking multiple beacons are possible, including for example, timedivision multiple access, code division multiple access, frequencyhopping, or other schemes for achieving a desired channelization.Similarly, a large number of different locators 1220 may measure rangesto a particular beacon 1210. Local oscillator 1250 may be considered asa component of an individual receiver 1225 or 1227 or as a commonfrequency standard for a plurality of receivers 1225, 1227.

[0193] Phase detector 1281 accepts the first IF signal from firstreceiver 1225 and a second IF signal from second receiver 1227 andgenerates an output voltage proportional to a phase difference betweenthe first IF signal and the second IF signal. For purposes ofillustration and not limitation, one exemplary embodiment of phasedetector 1280 is an Analog Devices AD 8302. This particular phasedetector also yields an output proportional to a magnitude differencethat may help identify and correct for propagation anomalies and providea more accurate determination of range in some circumstances.

[0194] Range detector 1290 is included in locator 1220 and accepts aninput from a phase detector 1281 for determining range r between beacon1210 and locator 1220. The inventors used a Measurement ComputingCorporation PC-Card-DAS 16/16 A/D PCMCIA Card and a notebook computer toembody range detector 1290, but there are a great many ways one skilledin the RF arts could implement range detector 1290.

[0195] The present invention offers good performance for a desired rangeof operation approximately within ranges r between 0.05 λ and 0.50 λaway, and more optimal performance was achieved within a range r between0.08 λ and 0.30 λ where λ is the wavelength of electromagnetic signal1215 transmitted by beacon 1210. Higher performance implementations ofthe present invention may operate at ranges r less than 0.05 λ andgreater than 0.50λ.

[0196] Fixed Beacon-Mobile Locator Architecture

[0197]FIG. 13 is a schematic diagram illustrating a near-field rangingsystem configured according to a fixed beacon-mobile locatorarchitecture. In FIG. 13, a fixed beacon-mobile locator ranging system1300 includes a first beacon 1310 in a first known, fixed positiontransmitting a first electromagnetic signal 1315. A locator 1320receives first electromagnetic signal 1315 and determines a first ranger₁. A second beacon 1312 in a second known, fixed position transmits asecond electromagnetic signal 1317. Locator 1320 receives secondelectromagnetic signal 1317 and determines a second range r₂. A thirdbeacon 1314 in a third known, fixed position transmits a thirdelectromagnetic signal 1319. Locator 1320 receives third electromagneticsignal 1319 and determines a third range r₃. A fourth beacon 1316 in afourth known, fixed position transmits a fourth electromagnetic signal1321. Locator 1320 receives fourth electromagnetic signal 1321 anddetermines a fourth range r₄. Electromagnetic signals 1315, 1317, 1319,1321 may be substantially similar electromagnetic signals withsubstantially similar frequencies, or may be a variety ofelectromagnetic signals with different frequencies. Electromagneticsignals 1315, 1317, 1319, 1321 may be transmitted substantiallycontemporaneously or at different times. For example, beacon 1310 maysimultaneously transmit a low frequency signal suitable for a long rangeand a high frequency signal suitable for a short range. Using ranges r₁,r₂, r₃, r₄, locator 1320 can determine its position. For purposes ofexplanation and not for limitation, four beacons 1310, 1312, 1314, 1316have been illustrated. One beacon is sufficient to yield useful rangeinformation for some applications. Two beacons can yield a position intwo dimensions subject to an ambiguity, three beacons can yield anunambiguous position in two dimensions or an ambiguous position in threedimensions, and four beacons yield an unambiguous position in threedimensions. With additional beacons providing ranges, one can obtain amore accurate position for locator 1320 using multilateration techniquesknown to those skilled in the RF arts.

[0198] Locator 1320 can also convey range and other useful informationvia an optional data bus 1395 to a central controller 1399 for analysis.Central controller 1399 can then relay position or other information viadata bus 1395 back to locator 1320. A centrally coupled (i.e., coupledto all components of ranging system 1300) controller 1399 or locator1320 can coordinate frequency of operation or other operationalparameters of locator 1320 and beacons 1310, 1312, 1314, 1316. Suchcoordination may include operating at appropriate frequencies to avoidinterference or to obtain optimal range information. Coordination mayalso include scheduling time or duty cycle of operation. Coordinationmay further include control of transmit power for coexistence, signalsecurity, or other reasons.

[0199] Fixed beacon-mobile locator system 1300 is advantageous when onewishes to track a limited number of assets, or if one wishes position,location, navigation, or guidance information to be available at apotentially large number of mobile locations. Fixed beacon-mobilelocator system 1300 is suitable for providing a user (with a locator1320) with fast updates of position within an area around or throughoutwhich a plurality of beacons (e.g., beacons 1310, 1312, 1314, 1316) havebeen deployed. A variety of applications are possible. For purposes ofillustration and not for limitation, a few applications are listedbelow.

[0200] For example, fixed beacons 1310, 1312, 1314, 1316 may be deployedin and around a golf course, a lawn, a farm, or another area in whichprecision guidance of equipment is desired. Locator 1320 may be placedon a robotic tractor, mower, golf ball gatherer, harvester, fertilizer,or other equipment. Locator 1320 may be used in a guidance or navigationsystem for such equipment. Locator 1320 may also be used to keep trackof golf carts, or other assets. Locator 1320 may be used to assistgolfers or others in determining their location and in particular theirlocation relative to a golf hole or another landmark of interest.

[0201] Fixed beacons 1310, 1312, 1314, 1316 may be deployed in andaround a mall, store, museum, business, amusement park, urban area,park, wilderness area, harbor, lake, property, home, apartment oranother area or facility in which one wishes individuals or equipment tobe able to monitor their location or position. Locator 1320 may becarried by an individual so that an individual may monitor his or herown location or a location of another individual (such as a familymember, friend, or other individual of interest). Locator 1320 may alsobe carried by an individual so that an individual may determine theirlocation relative to a landmark or other point or points of interest.Locator 1320 may be incorporated in a device that provides a user withlocation-specific information such as a price or other informationpertinent to a nearby object for sale, review, or evaluation. Locator1320 may be incorporated in a device that provides a user withlocation-specific information describing a nearby attraction, display,exhibit, hazard, or other feature of potential interest.

[0202] Locator 1320 may be incorporated into a vehicle to provideposition, guidance, or navigation information. An example is a precisionguidance or navigation system for aircraft such as unmanned aerialvehicles (UAV), boats, automobiles, unmanned ground vehicles (UGV) orother vehicles.

[0203] Fixed/Mobile Locator-Mobile Beacon Architecture

[0204]FIG. 14 is a schematic diagram illustrating a near-field rangingsystem configured according to a fixed/mobile locator-mobile beaconarchitecture. In FIG. 14, a fixed/mobile locator-mobile beacon rangingsystem 1400 includes a mobile beacon 1410 transmits a firstelectromagnetic signal 1415, a second electromagnetic signal 1417, athird electromagnetic signal 1419, a fourth electromagnetic signal 1421,and a fifth electromagnetic signal 1423. Electromagnetic signals 1415,1417, 1419, 1421, 1423 may be substantially similar electromagneticsignals with substantially similar frequencies, or a variety ofelectromagnetic signals with different frequencies. Electromagneticsignals 1415, 1417, 1419, 1421, 1423 may be transmitted at asubstantially similar time or at different times. For example, mobilebeacon 1410 may simultaneously transmit a low frequency signal suitablefor a long range and a high frequency signal suitable for a short range.

[0205] A first fixed locator 1420 receives first electromagnetic signal1415 and determines a first range r₁. A second fixed locator 1422receives second electromagnetic signal 1417 and determines a secondrange r₂. A third fixed locator 1424 receives third electromagneticsignal 1419 and determines a third range r₃. A fourth fixed locator 1426receives fourth electromagnetic signal 1421 and determines a fourthrange r₄. A fifth mobile locator 1428 receives fifth electromagneticsignal 1423 and determines a fifth range r₅. For purposes ofillustration, fifth mobile locator 1428 is shown as a directionallocator of the sort described as directional locator 1020 (FIG. 10), butfifth mobile locator 1428 could as readily be an omni-directionallocator of the sort described as omni-directional locator 820 (FIG. 8).

[0206] For purposes of explanation and not for limitation, four fixedlocators 1420, 1422, 1424, 1426 and one mobile locator 1428 areillustrated in FIG. 14. A single locator is sufficient to yield usefulrange information for some applications. For example, a single mobilelocator 1428 can enable a user to ascertain range r₅ from mobile beacon1410, thus allowing the user to home in on mobile beacon 1410. Twolocators can yield a position in two dimensions subject to an ambiguity,three locators can yield an unambiguous position in two dimensions or anambiguous position in three dimensions, and four locators yield anunambiguous position in three dimensions. With additional locatorsproviding ranges one can obtain a more accurate position for beacon 1410using multilateration techniques known to those skilled in the RF arts.

[0207] When a data bus 1495 is included in ranging system 1400, locators1420, 1422, 1424, 1426, 1428 may transmit ranges r₁, r₂, r₃, r₄, r₅ viadata bus 1495 to a central controller 1499 or another device (not shownin FIG. 14) connected to data bus 1495. Central controller 1499 cangather ranges r₁, r₂, r₃, r₄, r₅, calculate a position of beacon 1410,and relay that position information to any other device connected todata bus 1495.

[0208] Central controller 1099 (or another device connected to data bus1495) can coordinate a frequency of operation or other operationalparameters of mobile beacon 1410 and locators 1420, 1422, 1424, 1426,1428. Such coordination may include operating at appropriate frequenciesto avoid interference or to obtain optimal range information.Coordination may also include scheduling time or duty cycle ofoperation. Coordination may further include control of transmit powerfor coexistence, signal security, or other reasons.

[0209] Ranging system 1400 is particularly well configured for trackinglarge numbers of assets including, for example, tracking people orassets from a central location. A variety of applications are possible.For purposes of illustration and not for limitation, a few applicationsare listed below.

[0210] For example, a plurality of fixed locators (e.g., locators 1420,1422, 1424, 1426) may be deployed in and around a particular area ofinterest within which one wishes to track a plurality of beacons (e.g.,beacon 1410) attached to assets of interest. Ranging system 1400 is wellsuited for tracking cars, rental equipment, parts, components, tools orother assets in a manufacturing facility, a retail lot, warehouse, hold,vehicle, cargo container, storage area, hospital, or other facility inwhich one desires to track assets. A respective mobile beacon 1410 maybe placed in each car, piece of rental equipment, part, component, tool,or other asset whose location is desired to be known. If a respectivemobile beacon 1410 is removed from an area in and around which aninfrastructure of fixed locators have been placed, then a mobile locator(e.g., mobile locator 1428) may be used to help locate the wanderingmobile beacon 1410. This functionality is of particular utility if awandering mobile beacon 1410 is attached to stolen property. A locatorsuch as locator 1420 may be associated with a traffic signal, tollbooth, or other traffic related infrastructure and may monitor arespective mobile beacon 1410 in an approaching emergency vehicle, bus,or car thus allowing precision control of a traffic signal, or othermonitoring of the situation. It is useful to note here thatelectromagnetic signals associated with ranging system 1400 may bemodulated to include information, such as identifying informationrelating to an asset to which a mobile beacon is attached. In suchmanner, various assets bearing respective mobile beacons 1410 may beindividually identified or authenticated within ranging system 1400.

[0211] Further, a plurality of fixed locators (e.g., locators 1420,1422, 1424, 1426) may be deployed in and around a particular area ofinterest within which one wishes to track a plurality of beacons (e.g.,beacon 1410) attached or associated with people. Thus, ranging system1400 is well suited for tracking emergency responders such asfirefighting, police, SWAT team members, and medical personnel at anincident scene. Ranging system 1400 can be used to track employees in ahazardous environment like miners in a mine, workers at a facility wherehazardous materials are present, or corrections officers or prisoners ina prison. Ranging system 1400 may also be used to track patients,doctors, or other key personnel or equipment in a hospital, nursinghome, or other institution.

[0212] In still another exemplary application, ranging system 1400 maytrack skiers at a ski area, allowing skiers to be readily located evenin case of an avalanche or other emergency. Similar applications includetracking hikers, climbers, skydivers, hunters, fishermen, outdoorsmen,and others who engage in potentially dangerous activities and mightrequire rescue or assistance.

[0213] Patrons may be tracked at an amusement park, museum, festival,sporting event, convention, meeting, or other assembly drawing crowds.Sports competitors such as football players, soccer players, baseballplayers, swimmers, runners, and participants in other sports may havetheir positions monitored to assist in officiating, coverage, oranalysis of a sporting event. Sporting equipment or animals might betracked, including, by way of example and not by way of limitation,footballs, baseballs, soccer balls, rugby balls, race cars, yachts,thoroughbreds, or greyhounds.

[0214] Key personnel may be located in a business or other facility.Children and others requiring supervision may be monitored around ahome, neighborhood, school, campus, or other facility. Ranging system1400 is also applicable to a personal emergency response system (PERS),allowing rescuers to quickly locate an individual in need of assistance,such as a patient who has wandered away from a nursing home. Prisonersmay be tracked as part of a home release or other low securitysupervision program. Persons subject to restraining orders or otherrestrictions on their movements may be monitored to prevent theirviolating terms of their restrictions. A mobile locator (e.g., mobilelocator 1428) can be used to help find a person who has left an area inand around which an infrastructure of fixed locators (e.g., fixedlocators 1420, 1422, 1424, 1426) have been placed.

[0215] Ranging system 1400 may also be used to track a pet as part of apet containment system, or to allow an owner to monitor a pet'slocation. Wildlife may be tracked as part of a conservation project,research effort, or for other reasons. Ranging system 1400 may also beused to track and monitor livestock or other domesticated animals.

[0216] Reciprocal Beacon-Locator

[0217]FIG. 15 is a schematic diagram illustrating a near-field rangingsystem configured according to a reciprocal beacon-locator architecture.In FIG. 15, a reciprocal beacon-locator ranging system 1500 includes afirst beacon-locator 1520 and a second beacon locator 1522. Firstbeacon-locator 1520 transmits a first electromagnetic signal 1515.Second beacon-locator 1522 receives first electromagnetic signal 1515and calculates a range r from first beacon-locator 1520. Secondbeacon-locator 1522 may also transmits a second electromagnetic signal1517. First beacon-locator 1520 receives second electromagnetic signal1517 and calculates range r. If first beacon-locator 1520 and secondbeacon-locator 1522 are connected via an optional data bus 1595, thenfirst beacon-locator 1520 can trigger second beacon-locator 1522 to sendsecond electromagnetic signal 1517 so that first beacon-locator 1520 candetermine range r. For purpose of illustration and not for purpose oflimitation only two beacon-locators are shown. In some applicationshowever, it may be advantageous to have additional beacon-locators sothat each member of a larger group may track or be tracked.

[0218] A variety of applications are appropriate for ranging system1500. For purposes of illustration and not for limitation, a fewapplications are listed below. Reciprocal beacon-locator system 1500 isuseful in conjunction with two-way radios whose users desire to know howfar away a communicating party is situated. One may also advantageouslyincorporate a beacon-locator 1520, 1522 in devices that allow aplurality of people to find each other, such as parents and children atan amusement park, hunters, fishermen, or other outdoorsmen, or otherdevices in which combined tracking and communication within and amongmembers of a group is desired. Such a combined tracking andcommunicating arrangement may be useful not only for people, but alsofor vehicles, particularly aircraft and ships which may need to maintainparticular spacing or stations within a moving group. If a means fordirection finding is also used in a particular application, then bothrange and bearing information may be obtained. Reciprocal beacon-locatorsystem 1500 is also useful for allowing members of a team to monitoreach other's positions when visibility is impaired by smoke or otherintervening walls or objects. Further, reciprocal beacon-locator system1500 may be employed beneficially as part of a communication securitysystem that uses range or position information to validate orauthenticate the identity of a communicating party.

[0219] Passive TagArchitecture

[0220]FIG. 16 is a schematic diagram illustrating a near-field rangingsystem configured employing a passive tag architecture. In FIG. 16, apassive tag ranging system 1600 includes a locator 1620 equipped with aninterrogator antenna 1638 that radiates an interrogatory electromagneticsignal 1616. In alternate embodiments, the function of interrogatorantenna 1638 may be performed by a first magnetic antenna 1631, a secondmagnetic antenna 1633, or an electric antenna 1632. Interrogatoryelectromagnetic signal 1616 is detected by an interrogatory antenna 1639of a passive tag 1629. Passive tag 1629 collects energy frominterrogatory electromagnetic signal 1616 and re-radiates the collectedenergy as an electromagnetic signal 1617 via a passive tag transmitantenna 1635.

[0221] Interrogatory electromagnetic signal 1216 may have a differentfrequency or other different properties from re-radiated electromagneticsignal 1617. Although interrogatory antenna 1639 and passive tagtransmit antenna 1635 are shown as magnetic antennas they may beembodied in electric antennas. Further, passive tag 1629 may includeactive means to modulate re-radiated electromagnetic signal 1617.Electromagnetic signal 1617 is detected by first magnetic antenna 1631,second magnetic antenna 1633, and electric antenna 1632. Locator 1620then determines range r and possibly a bearing to passive tag 1629,using the near-field distance measurement teachings of the presentinvention.

[0222] Passive tag ranging system 1600 is a good product solution when alow cost but high volume implementation is an important goal. Passivetag 1629 may be attached to luggage, mail, assets for inventory controlor theft prevention, identification cards or other personal artifacts,or a wide variety of other people or assets whose location is desired tobe known with great precision.

[0223] A variety of neighboring passive tags 1629 may be distinguishedfrom each other by responsiveness to different interrogatoryelectromagnetic signals 1616 or by various modulations applied torespective transmitted electromagnetic signals 1617.

[0224] Near-Field Remote Sensing Architecture

[0225]FIG. 17 is a schematic diagram illustrating a near-field rangingsystem configured employing a near-field remote sensing architecture. InFIG. 17, a near-field remote sensing ranging system 1700 includes aremote near-field sensor 1720 is equipped with an interrogator antenna1738 that radiates an interrogatory electromagnetic signal 1716. Inalternate embodiments, the function of interrogator antenna 1738 may beperformed by a first magnetic antenna 1731, a second magnetic antenna1733, or an electric antenna 1732. Interrogatory electromagnetic signal1716 is incident on a remotely sensed object 1719. A reflectedelectromagnetic signal 1717 results when an incident interrogatoryelectromagnetic signal 1716 reflects from remotely sensed object 1719.The properties of reflected electromagnetic signal 1717 are dependentupon the electrical and geometric properties of remotely sensed object1719 as well as upon range r between near-field sensor 1720 and remotelysensed object 1719. Reflected electromagnetic signal 1717 is detected byfirst magnetic antenna 1731, second magnetic antenna 1733, and electricantenna 1732. Near-field sensor 1720 can evaluate reflectedelectromagnetic signal 1717 to infer properties of remotely sensedobject 1719.

[0226] Near-Field Ranging Method

[0227]FIG. 18 is a flow diagram illustrating the method of the presentinvention. A method 1800 for measuring distance between a first locusand a second locus begins at a START block 1802. Method 1800 continueswith transmitting an electromagnetic signal from the first locus, asindicated by a block 1804. Method 18000 continues with receiving theelectromagnetic wave at the second locus; the second locus being withinnear-field range of the electromagnetic signal, as indicated by a block1806. Method 1800 continues with, in no particular order, (1) detectinga first characteristic of the electromagnetic signal, as indicated by ablock 1808; and (2) detecting a second characteristic of theelectromagnetic signal, as indicated by a block 1810. Method 1800continues with measuring a difference between the first characteristicand the second characteristic, as indicated by a block 1812. Method 1800continues with employing the difference measured as represented by block1812 to calculate the distance between the first locus and the secondlocus, as indicated by a block 1814. Method 1800 terminates as indicatedby an END block 1816.

[0228] Fixed beacon-mobile locator ranging system 1300 (FIG. 13), afixed/mobile locator-mobile beacon ranging system 1400 (FIG. 14),reciprocal beacon-locator ranging system 1500 (FIG. 15), passive tagranging system 1600 (FIG. 16) and near-field remote sensing rangingsystem 1700 (FIG. 17) are presented for illustration and not forlimitation. A variety of alternate configurations and combinations ofarchitectures are also possible. For example, fixed locators 1420, 1422,1424, 1426 (FIG. 14) may be embodied in a beacon-locator configuration,such as beacon-locator 1520 (FIG. 15). Fixed locators 1420, 1422, 1424,1426 (FIG. 14) may be configured to cooperatively self-survey their ownrespective positions to enable rapid deployment of a positioning,locating, or tracking system. The specific exemplary applicationsprovided in connection with each respective ranging system architecturedescribed herein should not be interpreted as precluding use of adifferent architecture for a given respective exemplary application.

[0229] In another example, passive tag 1629 (FIG. 16) may be used with anetwork of locators (e.g., fixed locators 1420, 1422, 1424, 1426; FIG.14). In addition, nothing in this disclosure should be interpreted asprecluding a ranging, positioning or locating system from usingadditional information to refine an estimate of position. Such otherinformation may include, by way of example and not by way of limitation,a history of past positions or changes of position, or information fromother sensors or sources. In particular, the present invention is wellsuited as a supplement to a GPS type tracking system. The presentinvention can extend the functionality of a GPS type tracking andpositioning system into areas where GPS signals cannot penetrate or areunavailable. Also, the present invention may be used to achieve levelsof performance not attainable using GPS alone. Nothing in thisdisclosure should be interpreted as precluding use of the presentinvention in conjunction with any other prior art techniques fortracking, positioning, or locating. Similarly, the present invention maybe supplemented by prior art systems to improve the performance of thepresent invention in areas or at ranges where the present inventionalone may not yield reliable results.

[0230] Although this disclosure has focused on a single polarization inthe interest of simplicity in explaining the present invention, itshould be understood that the teachings of the present invention can bereadily extended to multiple polarization or polarization diversesystems with multiple parallel receive channels, including systemsemploying circular polarization. Various polarization capabilitiespermit the systems taught by the present invention to accommodate avariety of orientations between a beacon or passive tag and a locator.

[0231] To aid understanding the present invention, this disclosure hasfocused on a narrowband continuous wave (CW) implementation of thepresent invention. It should be understood that the present inventionmay also be implemented using multiple frequencies, time domain impulsewaveforms, stepped or swept sets of appropriate frequencies, or othersignals more complicated than an individual narrowband CW signal. Forexample, a phase difference of a CW signal may be related to a timedelay, or more generally, a Hilbert transform of an arbitrary timedomain signal. Any waveform (whether a CW waveform, short pulse,impulse, or time domain waveform, chirped waveform, or other waveform)will evolve from a near-field shape to a far-field shape in a mannerthat facilitates distance measurement and positioning according to theteachings of the present invention.

[0232] Specific applications have been presented solely for purposes ofillustration to aid the reader in understanding a few of the great manycontexts in which the present invention will prove useful. It shouldalso be understood that, while the detailed drawings and specificexamples given describe preferred embodiments of the invention, they arefor the purposes of illustration only, that the apparatus and method ofthe invention are not limited to the precise details and conditionsdisclosed and that various changes may be made therein without departingfrom the spirit of the invention which is defined by the followingclaims:

We claim:
 1. A system for measuring distance between a first locus and asecond locus; the system comprising: (a) at least one beacon device; arespective beacon device of said at least one beacon device beingsituated at said first locus and transmitting a respectiveelectromagnetic signal; and (b) at least one locator device; arespective locator device of said at least one locator device beingsituated at said second locus and receiving said respectiveelectromagnetic signal; said respective locator device being situated ata distance from said respective beacon device within near-field range ofsaid respective electromagnetic signal; said respective locator devicedistinguishing at least two characteristics of said respectiveelectromagnetic signal; said respective locator device employing said atleast two characteristics to effect said measuring.
 2. A system formeasuring distance between a first locus and a second locus as recitedin claim 1 wherein said at least two characteristics are a first signalcharacteristic proportional to a magnetic field component of saidelectromagnetic signal, and a second signal characteristic proportionalto an electric field component of said electromagnetic signal.
 3. Asystem for measuring distance between a first locus and a second locusas recited in claim 2 wherein said respective locator device effectssaid measuring by measuring a phase difference between said magneticfield component and said electric field component of saidelectromagnetic signal and employing said phase difference to determinesaid distance.
 4. A system for measuring distance between a first locusand a second locus as recited in claim 1 wherein selected said at leastone beacon device and selected said at least one locator device arecoupled in a unitary assembly.
 5. A system for measuring distancebetween a first locus and a second locus as recited in claim 2 whereinselected said at least one beacon device and selected said at least onelocator device are coupled in a unitary assembly.
 6. A system formeasuring distance between a first locus and a second locus as recitedin claim 1 wherein selected said at least one beacon device transmitssaid respective electromagnetic signal in response to receiving aninterrogating signal from a selected said at least one locator device.7. A system for measuring distance between a first locus and a secondlocus as recited in claim 1 wherein said at least one locator device isat least n locator devices effecting said measuring to a selected beacondevice of said at least one beacon device for ascertaining location ofsaid selected beacon device in n dimensions.
 8. A system for measuringdistance between a first locus and a second locus as recited in claim 4wherein said at least two characteristics are a first signalcharacteristic proportional to a magnetic field component of saidelectromagnetic signal, and a second signal characteristic proportionalto an electric field component of said electromagnetic signal, andwherein said respective locator device effects said measuring bymeasuring a phase difference between said magnetic field component andsaid electric field component and employing said phase difference todetermine said distance.
 9. A system for measuring distance between afirst locus and a second locus as recited in claim 6 wherein said atleast two characteristics are a first signal characteristic proportionalto a magnetic field component of said electromagnetic signal, and asecond signal characteristic proportional to an electric field componentof said electromagnetic signal, and wherein said respective locatordevice effects said measuring by measuring a phase difference betweensaid magnetic field component and said electric field component andemploying said phase difference to determine said distance.
 10. A systemfor measuring distance between a first locus and a second locus asrecited in claim 7 wherein said at least two characteristics are a firstsignal characteristic proportional to a magnetic field component of saidelectromagnetic signal, and a second signal characteristic proportionalto an electric field component of said electromagnetic signal, andwherein said respective locator device effects said measuring bymeasuring a phase difference between said magnetic field component andsaid electric field component and employing said phase difference todetermine said distance.
 11. An apparatus for effecting electromagneticranging; the apparatus comprising: (a) a transmitter device fortransmitting an electromagnetic wave; (b) a receiver device forreceiving said electromagnetic wave; said receiver device being situateda distance from said transmitter device within near-field range of saidelectromagnetic wave; said receiver device comprising a first receivermeans for detecting a first characteristic of said electromagnetic waveand a second receiver means for detecting a second characteristic ofsaid electromagnetic wave; (c) a measuring device coupled with saidreceiver device for measuring a difference between said firstcharacteristic and said second characteristic; and (d) a determiningdevice coupled with said measuring device for employing said differenceto calculate said distance.
 12. An apparatus for effectingelectromagnetic ranging as recited in claim 11 wherein said firstcharacteristic is a first signal characteristic proportional to amagnetic field component of said electromagnetic wave, and said secondcharacteristic is a second signal characteristic proportional to anelectric field component of said electromagnetic wave, and wherein saiddifference is a phase difference between said magnetic field componentand said electric field component; said determining device employingsaid phase difference to calculate said distance.
 13. An apparatus foreffecting electromagnetic ranging as recited in claim 11 whereinselected said transmitter device transmits said electromagnetic wave inresponse to receiving an interrogating signal from said receiver device.14. An apparatus for effecting electromagnetic ranging as recited inclaim 13 wherein said first characteristic is a first signalcharacteristic proportional to a magnetic field component of saidelectromagnetic wave, and said second characteristic is a second signalcharacteristic proportional to an electric field component of saidelectromagnetic wave, and wherein said difference is a phase differencebetween said magnetic field component and said electric field component;said determining device employing said phase difference to calculatesaid distance.
 15. A method for measuring distance between a first locusand a second locus; the method comprising the steps of: (a) transmittingan electromagnetic signal from said first locus; (b) receiving saidelectromagnetic wave at said second locus; said second locus beingwithin near-field range of said electromagnetic signal; (c) in noparticular order: (1) detecting a first characteristic of saidelectromagnetic signal; and (2) detecting a second characteristic ofsaid electromagnetic signal; (d) measuring a difference between saidfirst characteristic and said second characteristic; and (e) employingsaid difference to calculate said distance.
 16. A method for measuringdistance between a first locus and a second locus as recited in claim 15wherein said first characteristic is a first signal characteristicproportional to a magnetic field component of said electromagneticsignal, and said second characteristic is a second signal characteristicproportional to an electric field component of said electromagneticsignal, and wherein said difference is a phase difference between saidmagnetic field component and said electric field component.
 17. A methodfor measuring distance between a first locus and a second locus asrecited in claim 15 wherein said transmitting said electromagneticsignal occurs in response to receiving an interrogating signal.
 18. Amethod for measuring distance between a first locus and a second locusas recited in claim 17 wherein said first characteristic is a firstsignal characteristic proportional to a magnetic field component of saidelectromagnetic signal, and said second characteristic is a secondsignal characteristic proportional to an electric field component ofsaid electromagnetic signal, and wherein said difference is a phasedifference between said magnetic field component and said electric fieldcomponent.